AP Biology

Chromosomal Basis of Inheritance

Mitosis and Meiosis solved Cytologists worked out the process

of mitosis in 1875, and the process of meiosis in 1890s.

As cytologists and geneticists shared information, as well as with the development of better microscopes, the process became better understood.

Chromosome theory of inheritance

According to this theory, Mendelian genes have specific loci on chromosomes.

It is the chromosome that undergoes segregation and independent assortment.

Thomas Hunt Morgan THM was an embryologist at Columbia

University. He was the first to associate a specific gene with its location on a chromosome.

He studied Drosophila, or the fruit fly, because they were small, reproduced rapidly, and had unique characteristics that could be studied (they have only four pairs of chromosomes).

RED EYE

Wild-type vs. Mutant phenotypes

The normal phenotype for a character, or the phenotype most common in the population, is called the wild type. Ex) red eyes in Drosophila

Traits alternative to the wild type, such as white eyes, are called mutant phenotypes.

WHITE EYE

Sex-linked genes vs linked genes

Genes located on the sex chromosome are called sex-linked genes.

Genes located on the same chromosome tend to be inherited together. Such genes are said to be linked genes.

Genetic Recombination Genetic recombination is the production

of offspring with new combinations of traits inherited from two parents.

Meiosis and random fertilization generates genetic variation among offspring of sexually reproducing organisms.

Mendel’s Law of Independent Assortment illustrated this.

Recombination due to crossing-over

Parental types vs. recombinants

Offspring from a cross whose phenotype matches one of the parents is called a parental type.

Offspring who have different combinations of traits from the parents are called recombinants.

Linked genes & independent assortment

Linked genes do not assort independently because they are located on the same chromosomes and tend to move together through meiosis and fertilization.

Crossing-over accounts for recombination in linked genes. Occasionally the linkage between the genes is broken as homologous chromosomes exchange segments.

Gene Maps

One of Thomas Morgan’s students, Alfred Sturtevant, noticed a relationship between linked genes and recombination.

He developed a genetic map, which is an ordered list of the location of certain genes located along a particular chromosome.

He suggested that the farther apart two genes are on the chromosome, the higher the rate of recombination of those genes will be.

Linkage maps Linkage maps are genetic maps based

on recombination frequencies. Linkage maps do not exactly match the

actual chromosome because the frequency of crossing over is not uniform over the length of the chromosome.

Map units do not have absolute size, but can be used to show relative sequence of genes on the chromosome.

Constructing a genetic map The method used for mapping

genes on chromosomes assumes that the probability of crossing over between two genes is proportional to the distance between them.

Recombinations are converted to map units, and used to determine the sequence of genes.

Cytological maps Cytological maps locate genes with

respect to chromosomal features, such as stained bands.

Sex Chromosomes In humans, there are two varieties of

sex chromosomes, X and Y. Females have the genotype XX, and

males are XY. The sex of offspring is determined by

the male parent. Mother gives the offspring an X chromosome, and the father gives either an X or a Y chromosome.

SRY Gene Researchers have discovered a region

of the Y chromosome called the “sex-determining region of Y” or SRY gene.

It is responsible for the normal development of testes, and may be a biochemical trigger for the development of secondary sex characteristics in males.

Sex determination in animals

X-Y system: depends on whether sperm carries an X or Y chromosome Ex) humans and horses

X-O system: females are XX, males are XO Ex) crickets

Z-W system: sex chromosomes present in the ovum, males are ZZ and females are ZW Ex) hawks and other birds

Diploid-haploid: no sex chromosomes, females develop from fertilized ova (diploid), and males develop from unfertilized eggs (haploid) Ex) ants

X-linked alleles More males than females have disorders

inherited through sex-linked recessive alleles. Males have only one sex chromosome, so any

male receiving the recessive allele from his mother will express the trait.

Females will only express the trait if they receive the recessive allele from both parents.

For example, a girl will be color-blind only if she has a color-blind father and a mother who is a carrier.

Duchenne Muscular Dystrophy Effects 1/3500 males in the U.S. Caused by the absence of a key

muscle protein called dystrophin. Characterized by a progressive

weakening of the muscles and loss of coordination.

Hemophilia Caused by the lack of a certain

protein required for blood clotting. Hemophiliacs bleed excessively

when injured. Historical note: There was a high

incidence of hemophilia in the royal families of Europe throughout history, due to the intermarriage in families.

Why are calico cats female? The allele for gene color in cats is

located on the X chromosome, one for black, and one for orange.

Females can inherit two X chromosomes and both alleles, but males can only inherit one of the alleles.

XIST gene XIST is the X-inactive specific

transcript. May initiate X-inactivation.

Nondisjunction Nondisjunction is an accident in mitosis

or meiosis in which both members of a pair of homologous chromosomes or both sister chromatids fail to move apart properly.

In meiosis, this would result in one gamete receiving two of the same type of chromosome and another gamete receiving no copy.

Polyploidy Polyploidy describes the condition in which

an organism has more than two complete sets of chromosomes. Most common in plants, and can result in increased crop production.

Triploidy- diploid gamete joins with a normal haploid gamete, resulting in a 3n cell.

Tetraploidy- failure of a 2n zygote to divide after replicating its chromosomes.

Tetraploid mouse species

Chromosomal alterations Deletion- chromosomal segment is

removed. Duplication- segment is repeated. Inversion- reverses a segment

within a chromosome. Translocation- moves a segment

from one chromosome to another, nonhomologous chromosome

Down Syndrome (Trisomy 21) Results from nondisjunction during

gamete formation, so there is an extra chromosome 21 (trisomy)

Characterized by altered facial features, short stature, heart defects, susceptibility to respiratory infections and mental retardation.

Klinefelter’s Syndrome Males have an extra X

chromosome Individuals have male sex organs,

but the testes are abnormally small

Male is sterile

Turner Syndrome Females are XO. Must have an X

chromosome for embryo to survive.

Sex organs do not mature, and secondary sex characteristics fail to develop.

Females are sterile and usually short in stature.

Cri du Chat syndrome Deletion in chromosome five. Characterized by mental

retardation, small head with unusual facial features, and a cry that sounds like the mewing of a distressed cat.

Chromosomal translocation Chromosomal translocation is the

attachment of a fragment from one chromosome to another, nonhomologous chromosome.

Linked to the cause of chronic myelogenous leukemia, some cancers, and some cases of Down’s syndrome.

Genomic Imprinting The same alleles have different

affects on offspring depending on whether they arrive in the zygote from the egg or the sperm.

Parental affect on gene expression. Fragile X is most inherited from the

mother rather than the father.

Prader-Willi vs. Angelman syndrome

Both diseases are caused by the deletion of a segment of chromosome 15.

If the allele comes from the father, the disorder is Prader-Willi, and if from the mother the disorder is Angelman’s syndrome.

Prader-Willi- mental retardation, obesity, short stature, small hands and feet

Angelman’s- jerky movements, motor and mental deficits, sponataneous laughter at inappropriate times

Extranuclear genes Found on small circles of DNA in

mitochondria and chloroplasts (and other plastids).

Passed on to daughter organelles, codes for some of the organelle’s proteins

Maternal pattern of inheritance; mitochondria passed on by the zygote come from the cytoplasm of the ovum

Mitochondrial myopathy Mutation in the mitochondrial DNA coding

for proteins involved in ATP synthesis. Symptoms are related to muscle

functioning, where large numbers of mitochondria are found.

Symptoms include muscle weakness, intolerance of exercise, and muscle deterioration.

Mitochondrial myopathy may be associated with diabetes, heart disease, and Alzheimer’s disease as well.