UNIT FOUR: MEIOSIS, MEDELIAN GENETICS, AND HUMAN HEREDITY

MAIN IDEA: MEIOSIS PRODUCES HAPLOID GAMETES

OBJECTIVE 1: RELATE THE TERMS TRAITS AND GENES TO ONE ANOTHER AND DESCRIBE HOMOLOGOUS CHROMOSOMES

  1. Heredity is the passing on of characteristics from parents to offspring and the branch of biology that studies heredity is called genetics.
  2. Characteristics that are inherited are called traits.
  3. The instructions for each trait are located on chromosomes, which are found in the nucleus of a cell.
  1. The DNA on a chromosome is arranged in segments to control the making of proteins.
  2. These DNA segments are called genes and each chromosome is made of 100’s to 1000’S of genes, which determine the characteristics and function of the cell.
  1. Human body cells have 46 chromosomes, getting 23 from each parent.
  2. The chromosomes that make up one pair, one chromosome from each parent, are called homologous chromosomes.

1.Homologous chromosomes have the same length, centromere position, and the same genes for the same trait in the same order, although the traits might not be identical, i.e. earlobes (see page 270)

OBJECTIVE 2: DIFFERENTIATE BETWEEN HAPLOID AND DIPLOID CELLS.

  1. In order to maintain the same chromosome number from generation to generation, an organism makes gametes, sex cells with half the number of chromosomes.
  2. In humans our gametes have 23 chromosomes. All gametes are called haploid, a cell with ½ the number of chromosomes than a body or somatic cell. Haploid cells are represented by the variable n.
  3. When a haploid gamete combines with another haploid gamete, fertilization occurs.
  1. Fertilization results in a cell that will contain a total of 2n chromosomes
  2. 2n cells are called diploid.
  1. Each species has its own 2n number and the chromosome number of a species is not related to the complexity of the organism. When describing the number of pairs of chromosomes in an organism n can also be used.
  1. Humans have 23 pairs of homologous chromosomes where n equals 23.

MAIN IDEA: RECOGNIZE AND SUMMARIZE THE STAGES OF MEIOSIS

OBJECTIVE 3:EXPLAIN THE IMPORTANCE OF MEIOSIS THEN COMPARE AND CONTRAST MEIOSIS I AND MEIOSIS II

  1. Meiosis is a type of cell division which produces gametes containing half the number of chromosomes as a parent’s body cell, so it is referred to as reduction division. (see page 271)
  2. Meiosis consists of two separate cell divisions
  1. Only special diploid cells termed spermatogonia in the testis of the males and oogonia in the female ovary undergo meiotic divisions to produce haploid sperm and eggs.
  2. The first cell division, Meiosis I begins with one diploid cell (2n) cell and separates homologous chromosomes.
  3. By the end of the second cell division, Meiosis II, there are four haploid (n) cells.
  4. These haploid cells are called the sex cells or gametes. Male gametes are called sperm, while female gametes are called eggs.
  5. When the sperm fertilizes the egg the resulting cell called a zygote, has the diploid number of chromosomes.
  6. The zygote then develops by mitosis into a multicellular organism
  1. Sexual reproduction involves the production and subsequent fusion of haploid sex cells that will result in new combinations of alleles in a zygote.
  2. Meiosis I: (see pages 272 – 273)
  1. Interphase

a. The cell carries out metabolic processes, including the replication of DNA and protein synthesis

  1. Prophase I

a. replicated chromosomes are visible now and consist of two sister chromatids.

b. Homologous chromosomes condense and form pairs in a process called synapsis. The homologous chromosomes are physically bound together.

c. Crossing over: may occur between chromatids, pieces of homologous pairs are exchanged; this can lead to new combinations and more variety.

d.Centrioles migrate to poles, spindle fibers form and bind to centromeres.

  1. Metaphase I

a. homologous chromosomes line up in the middle, or equator of the cell.

  1. Anaphase I

a. homologous chromosomes separate from one another, guided by spindle fibers to opposite poles. At this point the chromosome number is reduced from 2n to n.

  1. Telophase I

a. Homologous chromosomes, each consisting of two sister chromatids, reach the opposite poles.

b.Each pole has only one member of the original pair of homologous chromosomes. Each chromosome still consists of two sister chromosomes.

c.Cytokinesis occurs, forming a furrow by pinching in animal cells and forming a cell plate in plant cells

d.In some species, the chromosomes uncoil, the nuclear membrane reappears, and nuclei reform.

  1. Meiosis II: occurs in both cells from Meiosis I, follows interkinesis, different than interphase because the DNA does not replicate.
  1. Prophase II

a. chromosomes are visible as they condense and spindle fibers from

  1. Metaphase II

a. a haploid number of chromosomes line up in the middle of the cell and the centromeres duplicate

  1. Anaphase II

a. sister chromatids are pulled apart at the centromere by the spindle fibers

b. sister chromatids begin to move toward opposite poles of the cell

  1. Telophase II
  2. chromosomes reach poles, forming two cells
  3. nuclear membrane and nuclei reform, completed with cytokinesis
  4. Results in four haploid cells. Each haploid cell contains one chromosome from each homologous pair. The haploid cells become the gametes.
  5. In males all four haploid cells become sperm cells
  6. In females, only one haploid cell becomes the egg; the other three, because of unequal cytoplasmic division, remain small, degenerate polar bodies which cannot be fertilized

OBJECTIVE 4: COMPARE AND CONTRAST MITOSIS AND MEIOSIS

A. Mitosis

1. One set of division phases

2. Produces two identical diploid cells

B. Meiosis

1. Consists of two sets of divisions

2. Produces four haploid daughter cells that are not identical

C. (see page 275)

OBJECTIVE 5: ANALYZE THE IMPORTANCE OF MEIOSIS IN PROVIDING GENETIC VARIATION

  1. Meiosis provides a mechanism for shuffling the chromosomes and the genetic information they carry. By shuffling chromosomes, genetic variation is produced. (see page 276)
  2. Combinations of gametes vary depending on how each pair of homologous chromosomes line up at Metaphase I, a random process.
  1. for example,

pea plants = n is equal to 7 so 27 = 128 eggs and 128 sperms cells

total number of different combinations is 128 x 128 = 16, 384.

  1. In humans,

Haploid cell (n) = 23 so 2 23 = 8, 388, 608

Total number of different combinations is 8,388,608 x 8,388,608 = 70 trillion!

  1. These numbers increase as the number of chromosomes in the species increase.
  1. Crossing over can occur anywhere at random on a chromosome.
  1. Usually 2 or 3 crossovers per chromosome during meiosis.
  2. Provides additional variation
  1. The reassortment of chromosomes and genetic information they carry, either by crossing over or independent segregation of homologous chromosomes is called genetic recombination.
  1. Genetic recombination is the major source of variation among organisms.
  2. Variation is the raw material that forms the basis for evolution.

MAIN IDEA: MENDEL EXPLAINED HOW A DOMINANT ALLELE CAN MASK THE PRESENCE OF A RECESSIVE ALLELE

OBJECTIVE 6: DESCRIBE MENDEL’S MONOHYBRID CROSSES AND APPLY THE TERMS HYBRID, P1, F1, and F 2

  1. Gregor Mendel - first person to succeed in predicting how traits would be transferred from one generation to the next.
  2. Mendel performed cross pollination by transferring male gametes from the flower of a true-breeding green seed plant to the female organ of a flower from a true-breeding yellow-seed plant (see page 278)
  1. The offspring produced were called hybrids; they came from parents that had different forms of a trait. (yellow vs. green peas)
  2. His crosses were called monohybrid; the parents differed by a single trait – seed color.
  1. To describe the generations of his monohybrid cross, Mendel used the following terms:
  1. P1 – the original parents
  2. F1 – offspring of the cross pollinated parents (F stands for “filial” or son or daughter).
  3. F2 – second filial generation; offspring of F1’s self-pollinating.
  4. Mendel studied seven traits and found the F1 generation plants to show a 3:1 ratio
  1. Mendel concluded that each organism has two factors that control each of its traits.
  1. These factors are called genes and are located on chromosomes. Genes are segments of DNA that code for a polypeptide or protein.
  2. Genes exist in alternative forms called allelesand are passed from generation to generation.
  3. An organisms two alleles are located on different copies of a chromosome – one inherited from the female parent and one inherited from the male parent. During fertilization, half of the chromosomes (DNA) come from one parent and the other half from the other parent.

OBJECTIVE 7: DESCRIBE MENDEL’S LAWS OF DOMINANCE AND AND APPLY THE TERMS DOMINANCE , RECESSIVE, PHENOTYPE, GENOTYPE, HOMOZYGOUS, HETEROZYGOUS

  1. Traits are described as either dominant or recessive.
  1. Dominant trait – the trait that is expressed (shows up) in the hybrid ex. Yellow (Y)
  2. Recessive trait - the trait hidden in the hybrid (not expressed) ex. green (y)
  1. The Law of Dominance - when an organism is a hybrid, (Yy) for a certain trait, only the dominant trait is expressed. (Y).
  2. When recording the results of crosses, use the same letter for different alleles of the same gene. An upper case letter is used for dominant traits and a lower case letter is used for the recessive traits.
  1. Tall (T) vs short (t), Round (R) vs. wrinkled (r), or Yellow (Y) vs green (y).
  1. Genotype – The gene combination and organism contains. It is established at fertilization and will determine the phenotype.
  1. Homozygous - two alleles for the trait are the same: RR, tt, YY
  2. Heterozygous – two alleles for a trait differ from each other: Rr, Tt, Yy
  1. Phenotype – The way an organism looks and behaves regardless of the genes it contains
  2. You can’t always know an organism’s genotype by looking at its phenotype.

OBJECTIVE 8: SUMMARIZE THE LAW OF SEGREGATION

A. Two alleles for each trait must separate during formation of gametes (eggs and sperm), and the parent passes on at random only one allele for each trait to each offspring.

B. (See page 279) During fertilization, two alleles for that trait unite to produce the genotype for that specific trait.

OBJECTIVE 9: DIFFERENTIATE BETWEEN A MONOHYBRID AND A DIHYBRID CROSS

A. A cross that involves hybrids for a single trait is called monohybrid cross.

1. During the self fertilization of the F1 generation, the male gametes (Yy) randomly fertilize the female gametes (Yy)

2. Such a cross results in the following genotypes: YY,Yy, yY, yy.. The genotypic ratio is 1YY: 2Yy: 1yy

3. Such a cross results in phenotypic ratio of 3 yellow seeds : 1 green seed.

B. Dihybrid Cross – considers two different traits at the same time. Traits are inherited independently of one another. For example Round seeds (R) are dominant to wrinkled seeds (r) and Yellow seeds (Y) are dominant to green seeds (y).

  1. RRYY x rryy = RrYy (dihybrid), round and yellow for all the F1’s
  2. Cross the F1’s: RrYy x RrYy (dihybrid cross)
  1. first identify all the possible gametes: use the foil method: First, Outside, Inside, Last
  2. possible gametes are RY, Ry, rY, and ry

C. Law of Independent Assortment

1. Mendel concluded that a random distribution of alleles occurs during gamete formation. Genes on separate chromosomes sort independently during meiosis.

2. (see page 280, figure 10.11) The law of independent assortment is demonstrated in the dihybrid cross by the equal chance that each pair of alleles (Yy and Rr) can randomly combine with each other. So Rr will separate from Yy.

a.Alleles in  gamete  possible allele combinations

in parental cell formation In gametes

YYR ¼

yYr 1/4

RyR ¼

Ryr ¼

b. The random assortment results in 4 possible gametes, each of which is equally likely to occur.

c. He also counted four different phenol types and found a ratio of about 9:3:3:1

d. If the alleles for seed shape and color were inherited together, only two kinds of seeds would be produced: RY, ry

OBJECTIVE 10: BE ABLE TO USE A PUNNETT SQUARE TO WORK OUT THE POSSIBLE RESULTS OF VARIOUS TYPES OF GENETIC CROSSES. DISCUSS THE PROBABILITY OF EXPERIMENTAL RESULTS

  1. Punnett Square – A shorthand way of finding the expected proportions of possible genotypes and phenotypes in the offspring of a cross.
  2. Makes it easier to keep track of the possible genotypes involved in a cross.
  1. Takes into account that fertilization occurs at random.
  2. Steps:
  1. Identify all of the possible parent gametes and place the females on the left edge and the male gametes on the top edge.
  2. Put the possible allele combinations of the zygotes (fertilized egg) in the inner boxes:

Parents: Male = TT, Female = Tt

T

/

T

T

t

Genotypes = TT, Tt

Phenotypes = All Tall

Cross two heterogyotes:

Genotypes =

Phenotypes =

Cross a homozygous recessive with a heterozygote:

Genotypes =

Phenotypes =

  1. All the above crosses were Monohybrid because they only looked at one trait.
  1. A dihybrid cross uses a larger Punnett Square: four boxes on each side with a total of 16 boxes (see page 282)

1. Remember the dihybrid cross has a phenotypic ratio for the F2 generation was 9:3:3:1

  1. Punnett squares show the possible combinations and the likelihood that each will occur.
  2. Genetics follows rules of chance; in reality you don’t get the exact ratio
  3. Calculate the probability of an event occurring: desired outcomes/total # of outcomes
  1. probablity of getting tails in a coin toss is one in two chances or 1:2 or ½
  2. probability of getting either a dominant or recessive allele for a specific trait is ½
  1. Results predicted by probability are more likely to be seen when there are a large number of offspring.

OBJECTIVE 11: DISCUSS HOW MEIOSIS EXPLAINS MENDEL’S RESULTS

  1. The segregation of chromosomes in Anaphase I of meiosis explains Mendel’s observation that each parent gives one allele for each trait at random to each offspring, regardless of whether the allele is expressed.
  2. The segregation of chromosomes at random in Anaphase I in meiosis explains Mendel’s observation that factors, or genes, for different traits are inherited independently of one another.
  3. Today, Mendel’s laws of heredity form the foundation of modern genetics

MAIN IDEA: THE CROSSING OVER OF LINKED GENES IS A SOURCE OF GENETIC VARIATION

OBJECTIVE 12: EXPLAIN HOW GENE LINKAGE CAN BE USED TO CREATE CHROMOSOME MAPS

  1. The new combination of genes produced by crossing over and independent assortment is called crossing over

1. Review possible combinations of genes in Objective 5.

  1. Chromosomes contain many genes that code for proteins and genes that are located close to one another on the same chromosome are said to be linked. Linked genes usually travel together during gamete formation.

1. The linkage of genes on a chromosome results in an exception to Mendel’s law of independent assortment because the linked genes usually do not segregate independently. (see page 283, Fig. 10.14)

C. 1. From studies it was found that linked genes can separate during crossing over.

  1. Crossing over occurs more often between genes that are far apart than those that are close together. A drawing called a chromosome map shows the sequence of genes on a chromosome and can be created using cross over data.

1. Chromosome map percentages are not actual chromosome distances, but represent relative

positions of the genes.

2. The higher the crossover frequency, the farther apart the two genes are.

OBJECTIVE 13: DEFINE THE TERM NONDISJUNCTION AND DISCUSS ITS VARIOUS EFFECTS IN REGARDS TO POLYPLOIDY

  1. The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction. Occasionally, both chromosomes of a homologous pair move to the same pole of the cell.
  2. The result of nondisjunction is a change from the usual number of chromosomes in the diploid cell.
  3. Nondisjunction can have many forms:
  1. Trisomy – results when a gamete with an extra chromosome is fertilized by a normal gamete. The zygote will have an extra chromosome.
  1. Extra chromosome 21 in humans results in Down Syndrome
  1. Monosomy - results when a gamete missing a chromosome fuses with a normal gamete. The zygote lacks a chromosome. In humans, most zygotes with monosomy do not survive.
  1. An example of monosomy that is not lethal is Turner’s Syndrome. Females only have a single X chromosome instead of two.
  1. Polyploidy - involves a total lack of separation of homologous chromosomes results in zygotes that are 3n or 4n.
  1. Rare in animals and always lethal to the human zygote.
  2. Polyploidy occurs roughly in one in three species of plants and the fruits from these plants tend to be larger and healthier. Therefore is can be beneficial and of commercial value. Plant breeders have learned to artificially produce polyploid plants using chemicals that cause nondisjunction.
  3. Bread wheat (6n), oats (6n), sugar cane (8n)

MAIN IDEA: THE INHERITANCE OF A TRAIT OVER SEVERAL GENERATIONS CAN BE SHOWN AS A PEDIGREE

OBJECTIVE 14: DESCRIBE HUMAN GENETIC DISORDERS THAT ARE CAUSED BY INHERITANCE OF RECESSIVE ALLELES

  1. Most genetic disorders are caused by recessive alleles and in order to be expressed, the person would need to be homozygous recessive.
  2. An individual who is heterozygous for a recessive disorder is called a carrier.
  3. Many of these alleles are rare, but a few are common in certain ethnic groups. (see page 297, Table 11.2)
  4. Punnett squares can be used to calculate the chance that offspring will be born with some of these genetic disorders.

E. Cystic fibrosis

  1. The most common genetic disorder among white Americans.
  2. About 1 in 20 are carriers and 1 in 2000 children born to white Americans inherits the disorder.
  3. Defective protein in the plasma membrane results in the formation and accumulation of thick mucus in the lungs and digestive tract.
  4. Life expectancy is mid-twenties.

F. Albinism

1. Genes do not produce the normal amount of pigment called melanin.

2. Results in no color to skin, hair, and eyes (eyes are blue)

3. Associated with vision problems and skin is highly susceptible to UV damage.

4. There is no cure, but life span not affected.

G. Tay-Sachs

  1. Recessive disorder of the central nervous system in which there is an absence of an enzyme that normally breaks down a lipid produced and stored in tissues of the CNS. Causes accumulation of lipids in the cells.
  2. Life expectancy is approximately five years.
  3. Common in the U.S. among the Amish and those with Eastern European Jewish ancestors.
  1. Galctosemia
  1. Recessive alleles cause the absence of an enzyme that breaks down galactose.
  2. Accumulation of galactose causes enlarged liver, kidney damage and mental disabilities.
  3. There is no cure but those that are homozygous recessive should limit their intact of lactose and galactose in the diet by avoiding milk products.

OBJECTIVE 15: DESCRIBE HUMAN GENETIC DISORDERS THAT ARE CAUSED BY THE INHERITANCE OF SINGLE DOMINANT ALLELE