Chromosome basis of

Mendel’s laws

(Fig. 15-1)

Sex-linked inheritance(Fig. 15.3)

Gene linkage

A CBa b c

Alleles A and B linked together; not linked to C or c

Alleles a and b linked together; not linked to C or c

Genes A and B are on the same chromosome and in the same linkage group. Do not assort independently.

Gene C is on a different chromosome and is in a different linkage group. Assorts independently.

Evidence for linked genes in Drosophila (Fig. 15.4)

Two alternate hypotheses for gamete formation in heterozygotes

Genes not linked (on different pairs of homologues)

b+ b vg+ vg

b+vg+ b+vg bvg+ bvg

Genes linked(on same pair of homologues)

b+ b vg+ vg

b+vg+ bvg

b+

b+ b+

b

vg+ vg

b+

b+

b

b

vg+ vg+

vg+

vg+vg vg

vg

vg

vg

b b

Recombination due to

crossing over (Fig. 15.5)

+

Sturtevant’s Mapping Concept (1913)

• Gene for a particular trait has a specific locus on a chromosome; alleles of gene occupy corresponding loci on homologous chromosomes.

• Genes are arranged in a linear sequence on chromosomes.

• Recombination frequency can show gene spacing:– Low frequency genes closely spaced.

– High frequency genes widely spaced.

Chromosome mapping (Fig. 15.6)

Partial genetic map of a

Drosophila chromosome (Fig. 15.7)

Chromosome systems of sex determination

(Fig. 15.8)

Transmission of sex-linked recessive traits (Fig. 15.9)

X-Inactivation in female mammals

• In mammals only one X chromosome remains active in each cell during embryonic development.

• Inactivated X chromosome becomes a Barr body—a compact object along the inside of the nuclear envelope. Inactivation is random.

• Examples: 1.) female cats with patches of tan and black fur, and 2.) human females with patches of skin lacking sweat glands.

Meiotic nondisjunction

(Fig. 15.11)

Can happen in mitosis also

Nondisjuction and polyploidy

• Can result from nondisjunction in all chromosomes together during mitosis or meiosis. Produces daughter cells with multiple sets of chromosomes.

• Haploid gamete fertilizing diploid gamete produces triploid zygote.

• Nondisjunction of diploid cell produces single daughter tetraploid cell.

Alterations of chromosome structure (Fig. 15.12)

Down Syndrome

• Produces characteristic facial features, short stature, heart defects, susceptibility to respiratory infection, and various degrees of sterility and mental retardation.

• Caused by inheriting an extra chromosome 21.– Nondisjunction in gamete formation (trisomy 21).– Translocation of chromosome 21 to another

chromosome

Genomic imprinting

(Fig. 15.14)

Cytoplasmic inheritance

• Some genes are on DNA of mitochondria and plastids reproducing in the cytoplasm.

• Offspring inherit mitochondria and plastids only from the cytoplasm of the ovum, not from sperm or pollen.

• Examples: 1.) yellow patches on leaves, and 2.) mitochondrial myopathy.