Theoretical Genetics – Part 2

Co-dominance, Multiple Alleles, Sex Linked Genes and Pedigree Charts

Assessment Statement

4.3.3State that some genes have more than two alleles (multiple alleles)

4.3.4Describe ABO blood groups as an example of codominance and multiple alleles

4.3.5Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans

4.3.6State that some genes are present on the X chromosome and absent from the shorter Y chromosome

4.3.7Define sex linkage

4.3.8Describe the inheritance of colour blindness and hemophilia as examples of sex linkage

4.3.9State that a human female can be homozygous or heterozygous with respect to sex-linked genes

4.3.10Explain that female carriers are heterozygous for X-linked recessive alleles

4.3.11Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance

4.3.12Deduce the genotypes and phenotypes on individuals in pedigree charts

We saw in the previous section that if we take one trait, with different alleles, and cross the two, we will get a 3:1 ratio in terms of the phenotypes and a 1:2:1 ratio for the genotypes.

These are very broad generalizations, and can be affected by numerous factors.

In some cases, there are multiple alleles, that can be expressed and some traits are co-dominant. Some traits are linked to sex (male or female). Using these as a guide, we can determine a pedigree, or the inheritance of several traits over several generations. We will explain these next.

Co-dominance

Co-dominance is when both alleles are expressed simultaneously rather than one being dominant and the other being recessive in the phenotype.

We saw in our test crosses that one trait will over ride another, if it is heterozygous. In the above, they are both seen in the organism.

Remember – they are not both dominant. One is expressed with the other.

For example, in the common garden flower Snapdragon, or Antirrhunum, when red flower plants are crossed with white flower plants, the F1 plants all are pink. When the pink flower plants are crossed, the F2 plants are found to have the ratio of 1 red to 2 pink to 1 white. Why?

First, write out the genotypes and gametes. For co-dominance, the gene is the main letter and the allele is the superscript. Both are expressed in capital letters.

Then we go through the test cross just like a monohybrid cross before.

The pink colouration of the petals occurs because both alleles are being expressed in the heterozygote and two pigment systems are present, rather than just the dominant allele only. When third type of phenotype appears, and blends the phenotype to produce a heterozygous offspring it is called incomplete dominance.

Example: For short horn cattle, the mating of a red bull (not the drink) and a white cow produces a calf that is described as roan. Roan is intermingled red and white hair. This is an example of co-dominance. Predict the genotypic ratio and phenotypic ratio if a red bull mates with a roan cow.

Multiple Alleles

The genes introduced so far have two forms, or alleles. For example, tall or dwarf, red or white, wrinkled or smooth. This is what Mendel identified.

When the gene for one trait exists as only two alleles, and the alleles play according to Mendel’s Law of Dominance, there are 3 possible genotypes and 2 possible phenotypes. We saw this with the Yellow vs. Green, with the yellow being the dominant phenotype in both the homozygous form (YY) and the heterozygous form (Yy).

If you remember from earlier, if there are only 2 alleles, but three possible phenotypes, there must be co-dominance or incomplete dominance occurring.

With some genes, there are more than two possible alleles. Then there are 4 or more possible phenotypes for a particular trait. These are called multiple alleles.

BUT, VERY IMPORTANT: There may be multiple alleles within a population, but individuals have only two of the alleles. WHY?

ANSWER: The individuals only have two biological parents. We inherit half our genes from one and the other half from the other, so we end up with two alleles for every trait in our phenotype.

Example of Multiple Alleles – Blood Type

As we discussed earlier, there are 4 blood types: A, B, AB, and O.

There are three alleles the gene that determines blood type.

IA – codes for A blood

IB – codes for B blood

i - codes for O blood

(With multiple alleles, we choose a single capital letter to represent the locus at which the alleles may occur. The individual alleles are then represented by an additional single letter (usually a capital) in a superscript position.)

If you notice, the allele for O is recessive (i).

If you look at the three alleles, the possible combinations are as follows:

GENOTYPES
/ PHENOTYPES
IAIA
IAi / Type A
Type A
IBIB
IBi / Type B
Type B
IAIB / Type AB
ii / Type O
  • There are 6 different genotypes and 4 different phenotypes.
  • There are two genotypes for both A and B blood – either homozygous dominant or heterozygous
  • There is only one genotype for O blood, homozygous recessive.
  • What is the deal with AB blood? What is this an example of?

Answer the following questions below.

Multiple Alleles and Co-dominance
  1. For short horn cattle, the mating of a red bull and a white cow produces a calf that is described as roan. Roan is intermingled red and white hair. This is an example of co-dominance. Predict the genotypic ratio and phenotypic ratio of the following:
  2. A roan bull and a roan cow
  3. A roan bull and a white cow
  1. A woman with Type O blood and a man who is type AB are expecting a child. What are the possible blood types of the kid?
  1. What are the possible blood types of a child who’s parents are both heterozygous for B blood?
  1. What are the chances of a woman with AB blood and a man with A blood having a child with type O?
  1. Jill is type O. She has two older brothers with blood types A and B. What are the genotypes of her parents, with respect to blood type?
  1. One busy night in an understaffed maternity ward, four children were born. The babies were mixed up and it was not certain who each baby belonged to. Baby Joe was type A, Baby Moe was type B, Baby Zoe was type AB and Baby Melvin was type O. The parents were the following:

Mr. and Mrs. Jones were A and B

Mr. and Mrs. Gerber were O and O

Mr. and Mrs. Lee were B and O

Mr. and Mrs. Santiago were AB and O

The nurses were able to figure out which child belonged to which family. Deduce how this was done.

Sex Linked Traits and Pedigree Charts

What genetically determines who is a boy and who is a girl?

That’s right, it’s the X and Y chromosomes!

In humans, gender is determined by specific chromosomes, known as the sex chromosomes. Each of us has one pair of sex chromosomes (either XX or XY) along with the 22 autosomal chromosomes.

What are the chances of having a boy? And who determines this chance?

Egg cells produced by meiosis all carry an X chromosome, but 50% of sperms carry an X and 50% carry a Y chromosome. At fertilization, an egg cell may fuse with a sperm carrying an X chromosome, which will produce a female offspring. The egg may fuse with a sperm carrying a Y chromosome, leading to a male offspring. The gender of the offspring is determined by the male partner. Also, due to the distribution of X and Y chromosomes, we would expect equal numbers of male and female offspring to be produced by a breeding population of humans, and other mammals, over time.

Evidence of 50:50 split.

Human X and Y Chromosomes and the Control of Gender

Initially, male and female embryos develop identically in the uterus. At the seventh week of pregnancy, the sexes are starting to be determined. This is shown by the development of male genitalia, only if a Y chromosome is present.

Why is XX female and XY male? On the Y chromosome is the prime male-determining gene. This gene codes for a protein, the testis-determining factor (TDF). TDF functions as a molecular switch; on reaching the embryonic gonad tissues, TDF initiates the production of a relatively low level of testosterone. The effect of this hormone at this stage is to inhibit the development of female genitalia, and to cause the embryonic genital tissue to form testes scrotum and penis.

In the absence of a Y chromosome, the embryonic gonad tissue forms an ovary. Then, partly under the influence of hormone from the ovary, the female reproductive structures develop.

In order for this to happen, the X and Y chromosomes have to pair up. We know that homologous chromosomes pair up during meiosis. Since the female has XX, the two are similar, containing the same genes, but maybe different alleles (more on this later).

With the X and Y chromosomes, only a very small part of the X and Y have complimentary alleles and pair up during Meiosis. The two only pair up at a very small portion of the chromosome.

In summary, the short Y chromosome carries genes specific for male sex determination, for coding for TDF and sperm production. The X chromosome carries an assortment of genes, very few of which are concerned with sex determination.

Sex linkage and Sex Linked Traits

Although most of our traits are carried on our autosomes, there are some traits that are located on the sex chromosomes – however, because the Y chromosome does not code for all the traits on the X, we have an interesting situation. Therefore, the genes present on the sex chromosomes are inherited with the sex of the individual. They are said to be sex linked characteristics or traits.

Sex linkage – is a special case of linkage occurring when a gene is located on a sex chromosome (usually the X chromosome)

The difference between inheritance of genes on autosomal chromosomes and sex-linked genes

The inheritance of sex-linked genes is different from the inheritance of genes on autosomes, because the X chromosome is much longer than the Y chromosome and many of the genes on the X chromosome are absent from the Y). (See diagram above)

In a female, if the X chromosome carries a recessive form of a gene (one allele), the pair X chromosome will often carry the dominant allele. As a result, the recessive allele will not be expressed.

In a male (XY), an allele present on the X chromosome is most likely to be on its own, and will be apparent even if it is recessive. In other words, it does not have a pair, dominant allele to “override” the recessive allele.

In conclusion, a human female can be homozygous or heterozygous with respect to sex-linked characteristics, whereas males will have only one allele.

An individual with a recessive allele of a gene that does not have an effect on the phenotype (ie. heterozygous) is known as a carrier. They carry the allele but it is not expressed. Therefore, female carriers are heterozygous for sex-linked recessive characteristics.

What does this mean for males?

Uh, oh!! The unpaired alleles of the Y chromosome deal mostly with male structures and male functions. But, some diseases are controlled by recessive genes found on the X chromosome. If a male human Y is paired with the recessive gene on the X, the disease-triggering allele will be expressed. A female must be homozygous recessive for a sex-linked characteristic for the allele to be expressed. Sorry guys!!!

Some examples of recessive conditions controlled by genes on the X chromosome are: Duchenne Muscular Dystrophy, red-green colour blindness, and haemophilia. The gene for hairy ears is on the Y chromosome, but it is more of an annoyance than a problem!!!

Examples of Genotypes for Sex Linkage

NB: A female has one of three possible genotypes for a sex-linked trait: homozygous dominant, heterozygous and homozygous recessive.

Human males cannot be heterozygous, since they only have one copy of the allele.

Heterozygous females are carriers.

Example 1 – Colour Blindness

Existing Alleles / XB for normal vision / Normal vision but a carrier / Xb for colour blindness
A female can be: / XB XB / XB Xb / Xb Xb
A male can be: / XBY / XbY

The homozygous recessive in a female is very rare.

Example 2 – Haemophilia

Existing Alleles / XH for normal blood clotting / Normal but a carrier / Xh for haemophilia
A female can be: / XH XH / XH Xh / Xh Xh
A male can be: / XHY / XhY

**** Xh Xh is homozygous lethal, meaning that the individual would not exist. Haemophilia is largely a male disease. The only way for a female to be a haemophiliac is to be homozygous recessive. When this happens, the condition is usually fatal in the uterus, typically resulting in a natural abortion. If the female was born with this condition, she would bleed to death during her first menstrual cycle.

Test Cross Examples

  1. If a female carrier for red-green colour blindness had a child with colour blind male, what are the possible percentages of males and females and normal sight versus colour blindness in the F1 generation? (Give the phenotypes and genotypes)
  1. If a homozygous dominant female for the Duchenne MD gene has children with man with Duchenne MD, what will be the genotypes and phenotypes for the F1 generation?
  1. If each of the male offspring had children with a female who was heterozygous for the MD gene, what would the phenotypes and genotypes be for the F2 generation?
  1. A man and a woman both have normal vision, but a daughter has red-green colour blindness. The man files for divorce on the grounds of infidelity. Can genetics support his case or make him look foolish?

Pedigree Charts

A pedigree can be represented as a diagram showing the phenotypes of a trait that is inherited from generation to generation. The genotypes can then be determined or predicted from these charts. Look at the following example of a simple pedigree diagram.

The squares represent males and the circles represent females. A horizontal line joining a male and a female indicates that the couple is married or is capable of having children. A shaded square or circle means that this person has the trait being studied.

In this example, the parents had three children: one girl and two boys. The father exhibits the trait being followed (shaded) and none of the children show this trait (none are shaded).

The Roman numerals represent the generation. Each person in a generation can be numbered to show birth order (e.g., 1, 2, 3 and so on). For example, II-3 represents the third child born in the second generation. Moving up in the diagram shows ancestors; moving down shows descendants.

Try one!

Use a drawing program or pencil and paper to draw the pedigree chart for the following family:

One couple (first generation) has four children: two boys and two girls.

One of the girls gets married and has two boys.

Pedigree charts can be used to identify a genetic pattern of inheritance.

The trait being studied is earlobe attachment, which is an autosomal trait. Free lobes is dominant. Having attached earlobes is recessive.

The shaded individuals are homozygous recessive for the trait and have attached lobes.

Looking at the genotypes, can you see any assumptions that were made about certain individuals?

The diagram tells us that the female, I-2, has free earlobes. We are guessing that her genotype is heterozygous Ee, but it might also be homozygous dominant EE. Looking at her offspring, we see that she only had one child--a male with free lobes. That is not enough information to know her genotype for sure. If she has the genotype Ee, by chance all of her children could have free lobes (I-1 can only contribute the recessive gene because he is homozygous for attached lobes) just as they would if she had the EE genotype.
The male, II-2, must be heterozygous dominant Ee. He has free lobes, but we know that he is carrying the gene for attached lobes from his father. The female II-1 has attached lobes, so we know that she is homozygous ee.
The male, III-3, has the genotype Ee because he has free lobes. We know this definitely because we know with certainty his mother's and father's genotypes (see above).
Individual IV-1 has free lobes but cannot have genotype EE since her mother cannot give her a dominant allele. Therefore, her genotype has to be Ee.
What is the genotype of III-1? The pedigree chart shows it to be heterozygous Ee. Again, we don't have information to be sure. The genotype could be either EE or Ee. Either Ee or EE in III-1 would produce the child IV-1 with Ee. If we had information about the parents of III-1, we might be able to know, but as it is, we simply can't. You can do a simple Punnett square to show probabilities of offspring in any of these scenarios. In a pedigree, we look at phenotypes of actual offspring rather than possible combinations of genes.

Just keep these 5 Rules for Pedigree Analysis

  1. Every gamete carries exactly one allele for every gene. The exceptions are sex linked genes in males.
  2. Any individual with the recessive phenotype (appearance) must be homozygous recessive.
  3. Any individual with a homozygous recessive offspring must have at least one recessive allele.
  4. Any individual with a homozygous recessive parent must have at least one recessive allele.
  5. If you don’t know, don’t guess. Use the Punnett Square.

Study the next pedigree to follow the inheritance of colour-blindness in humans, a sex-linked trait. Keep in mind that there are various types of colour-blindness and that this chart only shows one type.