Inheritance of Traits

Inheritance of Traits

Gregory Mendel (1822-1884), the discoverer of the gene and the founder of genetics, wasan Augustan monk from Brunn, Austria. In his spare time, Mendel bred pea plants in the monastery gardens. Many pea plants had many different traits, or genetically inherited features of an organism; for example some were tall, some were short, some peas were smooth, and others were wrinkly. Mendel then tried to make hybrids, or crosses of the plants. He did this by snipping the male part of the plant to prevent "selfing" (pea plant can fertilize themselves). Then he dusted the female part with the desired "father." Hethen tied bags over the flowers to prevent stray pollen from getting into the flowers. Thus, he was able to control the parentage of each generation. His first discovery was that tall plants crossed with short ones produced tall offspring, not medium offspring. He then concluded that some genes were dominant and some were recessive. A dominant gene means it will always show up if present. A recessive gene will be covered by a dominant one, and therefore only show when there are only recessive genes. When he raised hybrids though, he found about 1/4 of them were short, but the other 3/4 were tall. He then concluded that genes are made of two distinct types, or alleles. A plant may have the same (AA, aa ) or different(Aa) alleles. Two plants with the same alleles are called homozygous. Two plants with different alleles are called heterozygous. If you look at the two alleles that an organism has, you are looking at the genotype. For example, the genotype of a heterozygous plant would be Aa. However, what an organism actually looks like is considered it’s phenotype. In the previous example, an Aa genotype would give the tall phenotype. Itwasn't until 1900 when Mendel's works were actually noticed. Three men working independently, Hugo DeVries, Erich Von Tsohermark and Carl Correns did some experiments and came out with the same results as Mendel. They didn't take credit for it, but announced that Mendel had had the same results and had done such testing first.

Thinking Critically: What is the difference between alleles and genes? How are they similar?

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Activity:Create notecards for the 8 bolded words. Use blank paper, and fold it into 8 sections. Then cut the sections. You may use real notecards if you have them.

Inheritance of Traits (continued)

Mendel’s Rules

Mendel is considered the “father” of modern genetics. As the father, he developed some important rules for how understanding exactly how traits are inherited.

1. The rule of unit factors:The rule of unit factors states that all genes come in pairs, as you learned in the last section. What are those pairs called?

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Example

2. The rule of dominance:The rule of dominance basically states that most genes have 1 allele that is dominant over another (the recessive allele).

Example:

3. The Law of Segregation:This law states that, during meiosis, the homologous pairs will split up, therefore segregating the two alleles into two separate cells.

Example

4. The Law of Independent Assortment:This law states that the chromosomes in the homologous pairs are not linked to other homologous pairs, and therefore they randomly go to either one cell or another.

Example

Punnett Squares: Those fun little window things

The basic naked p-square looks like a window pane :

When given enough info about two parent organisms, we can use this window pane topredict the genotypes & phenotypes of their offspring. Exciting, isn’t it?

Here are the basic steps to using a Punnett Square when solving a genetics question. After you get good at this you should never miss a genetic question involving the cross of two organisms.

BABY STEPS:
1. determine the genotypes of the parent organisms
2. write down your "cross" (mating)
3. draw a p-square
4. "split" the letters of the genotype for each parent & put them "outside" the p-square
5. determine the possible genotypes of the offspring by filling in the p-square
6. summarize results (genotypes & phenotypes of offspring)

Example: "Cross a short pea plant with one that is heterozygous for tallness".

Step 1:

Step 2:

Step 3:

Step 4:

Step 5:

Step 6:

Parent Pea Plants
("P" Generation) / Offspring
("F1" Generation)
Genotypes:
______/ Phenotypes:
______/ Genotypes:
/ Phenotypes:

Punnett Square Practice: Basic Monohybrid Crosses

P-square practice Question #1

Let's say that in seals, the gene for the length of the whiskers has two alleles. The dominant allele (W) codes long whiskers & the recessive allele (w) codes for short whiskers.

a) What percentage of offspring would be expected to have short whiskers from the cross of two long-whiskered seals, one that is homozygous dominant and one that is heterozygous?
b) If one parent seal is pure long-whiskered and the other is short-whiskered, what percent of offspring would have short whiskers?

P-square Practice Question #2

In purple people eaters, one-horn is dominant and no horns is recessive.

Draw a Punnet Square showing the cross of a purple people eater that is heterozygous with a purple people eater that does not have horns. Summarize the genotypes & phenotypes of the possible offspring.

P-square Practice Question #3

A green-leafed engeplant is crossed with a engeplant with yellow-striped leaves. The cross produces 185 engeplants, all withgreen leaves.

a.) Which trait is dominant? What did the original cross look like?

b.) Give the genotypes & phenotypes of the offspring that would be produced by crossing two of the green-leafed engeplants obtained from the initial parent plants.

P-square Practice Question #4

Mendel found that crossing wrinkle-seeded plants with pure round-seeded plants produced only round-seeded plants.

A.) What genotypic & phenotypic ratios can be expected from a cross of a wrinkle-seeded plant & a plant heterozygous for this trait (seed appearance)?

Incomplete & Codominance

In many ways Gregor Mendel was quite lucky in discovering his genetic laws. He happened to use pea plants, which happened to have a number of easily observable traits that were determined by just two alleles. And for the traits he studied in his peas, one allele happened to be dominant for the trait & the other was a recessive form. Things aren't always so clear-cut & "simple" in the world of genetics, but luckily for Mendel (& the science world) he happened to work with an organism whose genetic make-up was fairly clear-cut & simple.

INCOMPLETE DOMINANCE

If Mendel were given a mommy black mouse & a daddy white mouse & asked what their offspring would look like, he would've said that a certain percent would be black & the others would be white. He would never have even considered that a white mouse & a black mouse could produce a GREY mouse! For Mendel, the phenotype of the offspring from parents with different phenotypes always resembled the phenotype of at least one of the parents. In other words, Mendel was unaware of the phenomenon of INCOMPLETE DOMINANCE.

I remember Incomplete Dominance in the form of an example like so:
RED Flower x WHITE Flower ---> PINK Flower
With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.

It's like mixing paints, red + white will make pink. Red doesn't totally block (dominate) the white, instead there is incomplete dominance, and we end up with something in-between.

Performing Crosses

We can still use the Punnett Square to solve problems involving incomplete dominance. The only difference is that instead of using a capital letter for the dominant trait & a lowercase letter for the recessive trait, the letters we use are both going to be capital (because neither trait dominates the other). So the cross I used up above would look like this:

R = allele for red flowers
W = allele for white flowers
red x white ---> pink
RR x WW ---> 100% RW /

The trick is to recognize when you are dealing with a question involving incomplete dominance. There are two steps to this:

1) Notice that the offspring is showing a 3rd phenotype. The parents each have one, and the offspring are different from the parents.
2) Notice that the trait in the offspring is a blend (mixing) of the parental traits.

Sample Questions

1. A cross between a blue blahblah bird & a white blahblah bird produces offspring that are silver. The color of blahblah birds is determined by just two alleles.

a) What are the genotypes of the parent blahblah birds in the original cross?
b) What is/are the genotype(s) of the silver offspring?
c) What would be the phenotypic ratios of offspring produced by two silver blahblah birds?

2. The color of fruit for plant "X" is determined by two alleles. When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit. What are the genotypes of the parent orange-fruited plants?

CODOMINANCE

First let me point out that the meaning of the prefix "co-" is "together".
Cooperate = work together. Coexist = exist together. Cohabitat = habitat together.

Have we got it together?

The genetic gist to codominance is pretty much the same as incomplete dominance. A hybrid organism shows a third phenotype --- not the usual "dominant" one & not the "recessive" one ... but a third, different phenotype. With incomplete dominance we get a blending of the dominant & recessive traits so that the third phenotype is something in the middle (red x white = pink).

I remember codominance in the form of an example like so:
red x white ---> red & white spotted

In COdominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.

With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.

Performing Crosses

When it comes to punnett squares & symbols, it's the same as incomplete dominance. Use capital letters for the allele symbols. My example cross from above would look like so:

R = allele for red flowers
W = allele for white flowers
red x white ---> red & white spotted
RR x WW ---> 100% RW /

Example 1:

A very common phenotype used in questions about codominance is roan fur in cattle. Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together). A good example of codominance.

Example 2:

Another example of codominance is human blood type AB, in which two types of protein ("A" & "B") appear together on the surface of blood cells.

Sample Questions
1. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.

2. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?

3. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together).

a) What pattern of inheritence does this illustrate?
b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat?

Sex-Linked Inheritance

A person’s sex is controlled by the 23rd pair of chromosomes, or sex chromosomes. A person with an XX genotype is going to be a ______, whereas someone with an XY is going to be a ______. During meiosis, a female can only give an ______, but a male can give an _____ or a _____. Because a male will always make the same number of X and Y sperm, the odds of having a girl or a boy are ______.

Inheritance “issues”

If a gene is found only on the X chromosome and not the Y chromosome, it is said to be a sex-linked trait. Because the gene controlling the trait is located on the sex chromosome, sex linkage is linked to the gender of the individual. Usually such genes are found on the X chromosome. The Y chromosome is thus missing such genes (See Diagram above.). The result is that females will have two copies of the sex-linked gene while males will only have one copy of this gene. If the gene is recessive, then males only need one such recessive gene to have a sex-linked trait rather than the customary two recessive genes for traits that are not sex-linked. This is why males exhibit some traits more frequently than females.

Examples of Sex-linked Traits:

Red-green colorblindness

Male Pattern Baldness

Hemophilia

Duchenne Muscular Dystrophy

Performing Crosses

When performing crosses involving sex-linked traits, you use the X and Y as your letters, and give them superscripts for the alleles.

XC = allele normal sight
Xc = allele red/green colorblindness
Yo= no allele
XCXC and XCXc= Normal female
Xc Xc = Colorblind female
XCYo= Normal male
Xc Yo= Colorblind male /

Sample Questions:

1.)Red-Green Colorblindness is a sex-linked recessive inherited disorder. Cross a mom who is homozygous normal with a dad who has colorblindness. Will any of the children have colorblindness?

2.) Cross a mom who is heterozygous for colorblindness with a dad who does NOT have colorblindness. Will any of their children have colorblindness?

3.) Based on the last 2 problems, will the mom or dad pass colorblindness to a son? Explain.

MULTIPLE ALLELES: How blood types work

MULTIPLE ALLELES

Now, if there are 4 or more possible phenotypes for a particular trait, then more than 2 alleles for that trait must exist in the population. We call this "MULTIPLE ALLELES".

**There may be multiple alleles within the population, but individuals have only two of those alleles.**

An excellent example of multiple allele inheritance is human blood type. Blood type exists as four possible phenotypes: A, B, AB, & O.

There are 3 alleles for the gene that determines blood type.
(Remember: You have just 2 of the 3 in your genotype --- 1 from mom & 1 from dad).

The alleles are as follows:

ALLELE
A
B
i / CODES FOR
Type "A" Blood
Type "B" Blood
Type "O" Blood

Notice that, according to the symbols used in the table above, that the allele for "O" (i) is recessive to the alleles for "A" & "B".

With three alleles we have a higher number of possible combinations in creating a genotype.

GENOTYPES
AA
Ai / RESULTING PHENOTYPES
Type A
Type A
BB
Bi / Type B
Type B
AB / TypeAB
ii / Type O

Notes:

1. As you can count, there are 6 different genotypes & 4 different phenotypes for blood type.
2. Note that there are two genotypes for both "A" & "B" blood --- either homozygous (AA or BB) or heterozygous with one recessive allele for "O" (Ai or Bi).
3. Note too that the only genotype for "O" blood is homozygous recessive (ii).

THINK!!

And lastly, what's the deal with "AB" blood? What is this an example of? The "A" trait & the "B" trait appear together in the phenotype. Think think think ....

SAMPLE QUESTIONS

Complete on separate sheet of paper

1. A woman with Type O blood and a man who is TypeAB have are expecting a child. What are the possible blood types of the kid?

2. What are the possible blood types of a child whose parents are both heterozygous for "B" blood type?

3. What are the chances of a woman with TypeAB and a man with Type A having a child with Type O?

4. Determine the possible genotypes & phenotypes with respect to blood type for a couple who's blood types are homozygous A & heterozygous B.

5. Jill is blood Type O. She has two older brothers (who tease her like crazy) with blood types A & B. What are the genotypes of her parents with respect to this trait?

6. A test was done to determine the biological father of a child. The child's blood Type is A and the mother's is B. Dude #1 has a blood type of O, & dude #2 has blood type AB. Which dude is the biological father?

College Prep Biology: Unit V: GeneticsPage 1

Make a Baby Lab

INTRODUCTION: Mendel proposed the law of segregation which states…

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This activity is based on the law of segregation. As parents, you will be flipping coins to symbolize the randomness of heredity. You will each pass an allele to your offspring which will determine his or her phenotype. Be aware that there are many more genes controlling facial characteristics than what we will examine today. Please follow the directions carefully.

1. Choose which one of you is the mother ______and which is the father ______.
2. Flip a coin to see if the child will be a boy or girl. Heads = boy, tails = girl. ______.
3. Look at number 1 – Face Shape. Determine whether you and your partner each have either a round face or a square face. Record “round” or “square” in the first two columns of the chart (mom’s phenotype, dad’s phenotype).
4. From this you will be able to determine your GENOTYPE. In this activity, the following rules apply:

• If you have the DOMINANT phenotype, assume that you are HETEROZYGOUS when given an option.
• If you have the RECESSIVE phenotype, you will always be HOMOZYGOUS RECESSIVE.

Record each of your genotypes on the chart in the spaces marked mom’s genotype and dad’s genotype.

1. Now determine which allele you will pass down to your child. Do this by flipping a coin.

Heads = you pass down the dominant allele, tails = you pass down the recessive allele.

(NOTE: ONLY HETEROZYGOUS INDIVIDUALS NEED TO FLIP A COIN, HOMOZYGOUS PEOPLE ONLY HAVE THE RECESSIVE ALLELE SO THAT IS WHAT THEY WILLPASS DOWN!!)

1. CIRCLE the allele that you will be passing down(2 pts). Write the child’s genotype in the appropriate column and then find out the child’s phenotype by looking at the key.
2. Based on your data, carefully draw a facial portrait of your child. Assume that your child is between the ages of 3-5 in this portrait.
3. Turn in both charts along with your picture. Name the baby!!

Trait / Mom’s Phenotype / Dad’s Phenotype / Mom’s Genotype / Dad’s Genotype / Child’s Genotype / Child’s Phenotype

1. Face Shape

1. Chin Shape

1. Cleft Chin

1. Skin Color

1. Hair Type

1. Widow’s Peak

1. Eyebrow Color

1. Eyebrow thickness

1. Eyebrow Placement

1. Eye Color

1. Eye Distance

1. Eye Size

1. Eye Shape

1. Eye Position

1. Eyelashes

1. Mouth Size

1. Lips

1. Protruding Lip

1. Nose Size

1. Nose Shape

1. Nostril Shape

1. Earlobe Attach.

1. Darwin’s Point

1. Ear Size

1. Ear Pits

1. Hairy Ears

1. Cheek Freckles

1. Forehead Freckles

1. Dimples

Trait / Dominant / Heterozygous / Recessive
1. Face Shape /
Round (RR, Rr) /
Square (rr)
2. Chin Shape /
Very Prominent (EE, Ee) / Less Prominent (ee)
3. Cleft Chin / Present (AA, Aa) / Absent (aa)
4. Skin Color / Skin color is controlled by more than one gene. For this lab, assume that there are three gene pairs involved. The first pair of genes is AA, Aa or aa. The second pair – BB, Bb, and bb. And the third pair is CC, Cc, or cc.
You MAY have to flip 3 times if you are heterzygous for each gene (medium brown). ONLY flip when there is a choice. Your baby should have 6 letters.
Very dark black – AA/BB/CC
Very dark brown – AA/BB/Cc
Dark brown – AA/Bb/Cc
Medium brown – Aa/Bb/Cc
Light brown (olive skin) – Aa/Bb/cc
Light tan (tan in the summer) – Aa/bb/cc
Really white (burn in the summer) – aa/bb/cc

5. Hair Type / Curly (HH) / Wavy (Hh) / Straight (hh)
6. Widow’s Peak / Present (DD, Dd) / Absent (dd)
7. Eyebrow Color / Very Dark (HH) / Medium Dark (Hh) / Light (hh)
8. Eyebrow Thickness / Bushy (BB, Bb) / Fine (bb)
Trait / Dominant / Heterozygous / Recessive
9. Eyebrow Placement / Not Connected (NN, Nn) / Connected (nn)
10. Eye Color / Darker eyes are produced in the presence of more active dominant alleles. To determine the color of the eyes, assume that there are two gene pairs involved. You MAY have to flip twice if you have two heterozygous genes.
Dark brown – AA/BB
Medium brown – AA/Bb
Light brown (hazel) – Aa/BB
Dark blue / green – Aa/Bb
Medium blue / green – Aa/bb
Light blue / green – aa/Bb
Pale blue – aa/bb
11. Eye Distance Apart / Close together (EE) / Average (Ee) / Far Apart (ee)
12. Eye Size / Large (EE) / Medium (Ee) / Small (ee)
13. Eye Shape / Almond (AA, Aa) / Round (aa)
14. Eye Position /
Straight (TT, Tt) / Slanted (tt)
15. Eyelashes / Long (LL, Ll) / Short (ll)
16. Mouth Size / Long (MM) / Average (Mm) / Short (mm)
17. Lips / Thick (TT) / Average (Tt) / Thin (tt)
18. Protruding Lip / Very protruding (HH) / Slightly (Hh) / Absent (hh)
Trait / Dominant / Heterozygous / Recessive
19. Nose Size /
Large (NN) / Medium (Nn) / Small (nn)
20. Nose Shape /
Rounded (RR, Rr) / Pointed (rr)
21. Nostril Shape /
Rounded (RR, Rr) / Pointed (rr)
22. Earlobe Attachment / Free (FF, Ff) / Attached (ff)
23. Darwin’s Ear Point / Present (DD, Dd) / Absent (dd)
24. Ear Size / Large (LL) / Normal (Ll) / Small (ll)
25. Ear Pits / Present (PP, Pp) / Absent (pp)
26. Hairy Ears / Absent (HH, Hh) / Present (hh)
27. Cheek Freckles / Present (FF, Ff) / Absent (ff)
28. Forehead Freckles / Present (FF, Ff) / Absent (ff)
29. Dimples / Present (DD, Dd) / Absent (dd)

Family Heredity: Pedigrees

Imagine being able to trace your ancestors back more than 1,000 years. Many people who live in Iceland can do just that. Besides having extensive genealogy records, Iceland has an unusually isolated and homogeneous population making it valuable for genetic research. Scientists are examining the genes of Icelanders in hopes to identify certain disease genes.