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Genetics Unit 3: Chromosomes, Mitosis, Meiosis and Sex Linkage

Genes ride on chromosomes, so understanding how chromosomes behave in mitosis and meiosis explains Mendel's laws, sex determination and the special patterns of X-linked traits.

Covers: Chromosome structure and karyotypes, Mitosis and the cell cycle, Meiosis and sources of variation, Chromosome and DNA counts, Sex determination systems, X-linked inheritance, X inactivation and dosage compensation

Study guide

A eukaryotic chromosome is one long DNA molecule packaged with proteins. Before division, each chromosome is replicated into two identical sister chromatids joined at the centromere. Diploid cells contain homologous pairs: one maternal and one paternal chromosome carrying the same genes, possibly with different alleles. A karyotype displays a cell's chromosomes arranged by size and banding pattern; humans have 22 pairs of autosomes plus XX or XY. The C-value describes DNA amount: a G1 diploid cell is 2C, after S phase it is 4C, and a gamete is 1C.

Mitosis produces two genetically identical daughter cells. After replication in S phase, chromosomes condense in prophase, line up individually at the equator in metaphase, have their sister chromatids pulled apart in anaphase, and re-form nuclei in telophase. Meiosis has one replication and two divisions. In meiosis I, homologs pair (synapsis) in prophase I, cross over, line up as pairs at metaphase I and separate at anaphase I, halving the chromosome number. Meiosis II resembles mitosis: sister chromatids separate, giving four haploid cells.

Meiosis creates variation in two ways. Independent assortment comes from the random orientation of each homologous pair at metaphase I, giving 2ⁿ chromosome combinations (over 8 million for humans). Crossing over exchanges segments between non-sister chromatids at chiasmata, creating new allele combinations on single chromosomes. Errors also matter: nondisjunction, a failure of separation, produces gametes with extra or missing chromosomes. A meiosis I error affects all four products, while a meiosis II error in one cell leaves the other two gametes normal.

Sex determination varies. Mammals use XX/XY, and the Y-linked SRY gene triggers testis development, so XXY humans are male and X0 humans are female. Drosophila use the ratio of X chromosomes to autosome sets, so an XXY fly is female. Birds use ZW females and ZZ males, and many insects use XX/X0. Genes on the X show distinctive patterns because males are hemizygous: one X allele determines their phenotype. X-linked recessive traits are more common in males, pass from affected men through carrier daughters to grandsons, and are never passed from father to son.

Female mammals equalize X-gene dosage by inactivating one X in each somatic cell early in development. The choice is random and is kept by all descendant cells, so heterozygous females are mosaics, as seen in calico cats. The inactive X condenses into a Barr body, and the number of Barr bodies equals the number of X chromosomes minus one. Skewed inactivation can make some carriers show symptoms. X-linked dominant traits pass from an affected father to all daughters and no sons. Separate from sex linkage, sex-influenced traits are autosomal traits expressed differently in each sex, and sex-limited traits appear in only one sex.

Key ideas

• Sister chromatids are identical copies; homologs are maternal and paternal versions of a chromosome.
• Homologs separate in anaphase I; sister chromatids separate in anaphase II and in mitosis.
• Independent assortment and crossing over are the two meiotic sources of new allele combinations.
• Mammalian sex depends on SRY on the Y; Drosophila sex depends on the X:autosome ratio.
• Males are hemizygous for X-linked genes, so X-linked recessive traits appear more often in males.
• There is no father-to-son transmission of X-linked traits.
• Random X inactivation makes heterozygous females mosaics, with Barr bodies = X − 1.

Formulas and rules

Common mistakes

• Counting chromosomes and chromatids as the same thing after replication.
• Saying sister chromatids separate in anaphase I.
• Using 1/4 for the risk to a son when the sex is already known (it is 1/2 for a carrier mother).
• Thinking an affected father can pass an X-linked trait to his son.
• Assuming the Y determines sex in all species, including Drosophila.

Before the test, make sure you can

• Compare mitosis and meiosis by divisions, ploidy and genetic outcome.
• Count chromosomes, chromatids and DNA content at any stage.
• Explain how meiosis produces variation and calculate 2ⁿ combinations.
• Compare XY, X:A ratio, ZW and X0 sex determination.
• Predict offspring for X-linked recessive and dominant crosses.
• Explain X inactivation, Barr bodies and mosaicism.
• Determine the parent and meiotic division of a nondisjunction event.

Words to know

Sister chromatids

Two identical DNA copies of one replicated chromosome joined at the centromere.

Homologous chromosomes

A maternal and a paternal chromosome carrying the same genes in the same order.

Centromere

The constricted region where sister chromatids are held together and spindle fibers attach.

Karyotype

An ordered display of an individual's chromosomes.

Synapsis

The pairing of homologous chromosomes during prophase I.

Chiasma

A visible point where non-sister chromatids have crossed over.

Reductional division

Meiosis I, which separates homologs and halves the chromosome number.

Nondisjunction

Failure of chromosomes or chromatids to separate properly during division.

Hemizygous

Having only one copy of a gene, as males do for X-linked genes.

SRY

The Y-linked gene that triggers testis development in mammals.

X inactivation

Silencing of all but one X chromosome in each female mammalian somatic cell.

Barr body

A condensed, inactive X chromosome.

Dosage compensation

A mechanism that equalizes expression of sex-linked genes between sexes.

Sex-influenced trait

An autosomal trait expressed differently in males and females.

Sex-limited trait

A trait expressed in only one sex.

ZW system

Sex determination in birds where females are ZW and males ZZ.

Sample practice questions

30 more questions like these, from level 1 to 5, are in the practice set.