ANSC 221 (Exam 3): G7- Chromosomal Basis of Inheritance

What is the Chromosome Theory of Inheritance?

Genes are located at specific positions (loci) on chromosomes, and the behavior of chromosomes during meiosis accounts for inheritance patterns.

What is a karyotype?

A display of the chromosome pairs of a cell arranged by size and shape, prepared from somatic cells (diploid, 2n).

What determines sex in many species?

Sex is largely determined by the inheritance of sex chromosomes.

What are the two types of sex chromosomes in mammals?

X and Y chromosomes.

What is the difference between homogametic and heterogametic sexes?

Females are homogametic (XX), while males are heterogametic (XY).

What is the SRY gene?

The sex-determining region of Y chromosome required for the development of testes.

How do X-linked genes differ in inheritance between males and females?

Fathers pass X-linked alleles to all daughters and none to sons; mothers pass X-linked alleles to both sons and daughters.

What is a sex-linked gene?

A gene located on either sex chromosome.

What is the pattern of inheritance for X-linked recessive traits?

Males express the trait with one copy of the allele, while females need two copies to express the trait.

What is X inactivation?

In female mammals, one X chromosome in each cell becomes inactivated during early embryonic development.

What are linked genes?

Genes located near each other on the same chromosome that tend to be inherited together.

What is genetic recombination?

The production of offspring with combinations of traits that differ from those found in either parent.

What is crossing over?

The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis.

What is the recombination frequency?

The percentage of recombinant offspring produced from a genetic cross, which is always less than 50% for linked genes.

What happens during the independent assortment of unlinked genes?

The random orientation of homologous chromosomes leads to the independent assortment of genes located on different chromosomes.

What is the significance of the Y chromosome in inheritance?

Very few disorders are transmitted from father to son on the Y chromosome due to the low number of genes.

What is the role of gonads in sex determination?

Embryos have generic gonads until 2 months old, after which they develop into either testes or ovaries depending on the presence of a Y chromosome.

What is the difference between sex-linked and linked genes?

Sex-linked genes are located on sex chromosomes, while linked genes are located on the same chromosome and tend to be inherited together.

What is the impact of crossing over on inheritance?

Crossing over breaks the physical connection between specific alleles of genes on the same chromosome, leading to genetic variation.

Chromosome theory of inheritance

Genes are located at specific loci on chromosomes; their behavior during meiosis explains inheritance.

Law of segregation

Two alleles for each gene separate during gamete formation.

Law of independent assortment

Alleles of genes on nonhomologous chromosomes assort independently.

Karyotype

Display of chromosome pairs arranged by size and shape.

Diploid vs haploid

Diploid (2n = 46), Haploid (n = 23); autosomes + sex chromosomes.

Meiosis I and II

Homologous chromosomes separate in meiosis I; sister chromatids in meiosis II.

Sex chromosomes

X and Y in mammals; XX = female, XY = male.

Homogametic vs heterogametic

Females = homogametic (XX), males = heterogametic (XY).

X-Y pairing

Short homologous regions allow pairing during meiosis.

X-Y system

Mammals; sperm determines sex.

X-0 system

Insects; males have one X, females have two.

Z-W system

Birds; egg determines sex (ZW = female, ZZ = male).

Haplo-diploid system

Bees/ants; males = haploid, females = diploid.

SRY gene

Located on Y chromosome; triggers testis development.

WNT4 gene

Promotes ovary development in absence of SRY.

X-linked vs Y-linked

~1,100 X-linked genes; ~78 Y-linked genes.

Inheritance patterns

Fathers pass X-linked alleles to daughters only; mothers to both.

X-linked recessive traits

Males express with one copy; females need two.

Carrier females

XNXn

Affected males

XnY

Inheritance probabilities

Males more likely to inherit due to hemizygosity.

Dosage compensation

One X chromosome in females is inactivated.

Tortoiseshell cats

Mosaic fur color due to X inactivation.

Linked Genes

Genes close together on the same chromosome; inherited together.

Deviation from Mendel

Linked genes do not assort independently.

Unlinked genes

Independent assortment; 50% recombination frequency.

Example of recombination frequency

391 recombinants out of 2,300 offspring → 17% recombination frequency.

Recombination frequency formula

Recombination frequency = # of recombinants/total offspring