Genetics, Genetics, ContinuedContinued

Week 10Week 10

MeiosisMeiosis

Homologous- each chromosome from Homologous- each chromosome from the male parent has a corresponding the male parent has a corresponding chromosome from the female parentchromosome from the female parent

Diploid- a cell that contains both sets Diploid- a cell that contains both sets of homologous chromosomesof homologous chromosomes

Haploid- cells that contain only one Haploid- cells that contain only one set of chromosomesset of chromosomes

MeiosisMeiosis Meiosis is a process of reduction division Meiosis is a process of reduction division

in which the number of chromosomes per in which the number of chromosomes per cell is cut in half through the separation cell is cut in half through the separation of homologous chromosomes in a diploid of homologous chromosomes in a diploid cell.cell.

Meiosis I- results in two diploid daughter Meiosis I- results in two diploid daughter cells, each with the same number of cells, each with the same number of chromosomes as the original cell.chromosomes as the original cell.

Tetrad- structure formed by the pairing of Tetrad- structure formed by the pairing of homologous chromosomeshomologous chromosomes

Crossing-over- exchanging portions of Crossing-over- exchanging portions of chromatids while forming tetradschromatids while forming tetrads

Animation

MeiosisMeiosis

Meiosis II- results in four haploid Meiosis II- results in four haploid daughter cellsdaughter cells

Sperm- male haploid gametesSperm- male haploid gametes Egg- female haploid gametesEgg- female haploid gametes Mitosis results in the production of Mitosis results in the production of

two genetically identical diploid two genetically identical diploid cells, whereas meiosis produces four cells, whereas meiosis produces four genetically different haploid cells.genetically different haploid cells.

Gene MapsGene Maps

Chromosomes assort independently, Chromosomes assort independently, not individual genes.not individual genes.

A gene map shows the relative A gene map shows the relative locations of each known gene on a locations of each known gene on a chromosome.chromosome.

A karyotype is a picture of arranged A karyotype is a picture of arranged chromosomes.chromosomes.

Gene MapsGene Maps

Humans have 46 chromosomes. Two of Humans have 46 chromosomes. Two of the 46 are known as sex chromosomes. the 46 are known as sex chromosomes. Females have two copies of X, and Females have two copies of X, and males have 1 X and 1 Y.males have 1 X and 1 Y.

The remaining 44 are autosomes.The remaining 44 are autosomes. All human egg cells carry a single X All human egg cells carry a single X

chromosome. Half of all sperm cells chromosome. Half of all sperm cells carry an X chromosome and half carry carry an X chromosome and half carry a Y chromosome.a Y chromosome.

Gene MapsGene Maps A pedigree chart shows the relationships A pedigree chart shows the relationships

within a family. within a family. The average human gene has about The average human gene has about

3,000 base pairs. The largest gene in 3,000 base pairs. The largest gene in the human genome has more than 2 the human genome has more than 2 million base pairs.million base pairs.

Chromosome 21 and 22 are the smallest. Chromosome 21 and 22 are the smallest. Chromosome 22 problems can cause Chromosome 22 problems can cause leukemia and tumors. Chromosome 21 leukemia and tumors. Chromosome 21 problems can result in Lou Gehrig’s problems can result in Lou Gehrig’s disease (ALS).disease (ALS).

Sex-linked GenesSex-linked Genes

Sex-linked genes are found on the X or Y Sex-linked genes are found on the X or Y chromosome.chromosome.

Males have just one X chromosome. Thus, Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, all X-linked alleles are expressed in males, even if they are recessive.even if they are recessive.

Females randomly switch off one of the X Females randomly switch off one of the X chromosomes.chromosomes.

The most common error in meiosis occurs The most common error in meiosis occurs when homologous chromosomes fail to when homologous chromosomes fail to separate. This is called nondisjunction.separate. This is called nondisjunction.

NondisjunctionNondisjunction

If nondisjunction occurs, abnormal If nondisjunction occurs, abnormal numbers of chromosomes may find numbers of chromosomes may find their way into gametes, and a their way into gametes, and a disorder of chromosome numbers disorder of chromosome numbers may result.may result. Down syndrome- involves having 3 Down syndrome- involves having 3

copies of chromosome 21.copies of chromosome 21.

Natural SelectionNatural Selection Natural selection on single-gene traits can lead Natural selection on single-gene traits can lead

to changes in allele frequencies and thus to to changes in allele frequencies and thus to evolution.evolution.

Natural selection can affect the distributions of Natural selection can affect the distributions of phenotypes in any of three ways.phenotypes in any of three ways. Directional selection- individuals at one end of the Directional selection- individuals at one end of the

curve have higher fitness than individuals in the curve have higher fitness than individuals in the middle or at the other end.middle or at the other end.

Stabilizing selection- individuals near the center of Stabilizing selection- individuals near the center of the curve have higher fitness that individuals at the curve have higher fitness that individuals at either end.either end.

Disruptive selection- individuals at the upper and Disruptive selection- individuals at the upper and lower ends of the curve have higher fitness that the lower ends of the curve have higher fitness that the individuals in the middle.individuals in the middle.

Genetic DriftGenetic Drift

A random change in allele frequency A random change in allele frequency is called genetic drift.is called genetic drift.

In small populations, individuals that In small populations, individuals that carry a particular allele may leave carry a particular allele may leave more descendants than other more descendants than other individuals, just by chance. Over individuals, just by chance. Over time, a series of chance occurrences time, a series of chance occurrences of this type can cause an allele to of this type can cause an allele to become common in a population.become common in a population.

Hardy-Weinberg Hardy-Weinberg PrinciplePrinciple

The Hardy-Weinberg principle states that allele The Hardy-Weinberg principle states that allele frequencies in a population will remain constant frequencies in a population will remain constant unless one or more factors cause those unless one or more factors cause those frequencies to change.frequencies to change.

Genetic equilibrium- allele frequencies do not Genetic equilibrium- allele frequencies do not changechange

Five conditions are required to maintain genetic Five conditions are required to maintain genetic equilibrium from generation to generation:equilibrium from generation to generation: random matingrandom mating very large populationvery large population no movement into or out of the populationno movement into or out of the population no mutationsno mutations no natural selectionno natural selection