The Chromosomal Basis of Mendel’s Laws

Mendel’s “principle of segregation”Mendel’s “principle of segregation”a.pairs of genes on homologous

chromosomes separate during gamete gamete formation (meiosis) (meiosis) and end up in different gametes. and end up in different gametes.

b. the fusion of gametesgametes at fertilization pairs genes once again.Ex: The alleles for height in Mendel’s pea plants end up in separate gametes. Tt X Tt could yield four the offspring: TT, Tt, Tt, tt.

• Mendel’s “principle of independent Mendel’s “principle of independent assortment”assortment”

a. each pair of alleles on nonhomologous chromosomes segregates independently of other pairs during gamete formation

Ex: When Mendel crossed F1 plants that were heterozygous for

round yellow peas, some F2 plants

did not resemble the parent plants.

Morgan’s First Mutant

Sex-linked Inheritance

Drosophila TestcrossBlack and Vestigial

Wild Type (Gray and Normal)

Gray and Normal Black and Vestigial

Evidence for linked genes in Drosophila

Non-ParentalPhenotype

Recombination Due to Crossing Over

Recombination Due to Crossing Over

If the two genes were on different chromosomes, the alleles from the F1 dihybrid would sort into gametes independently, and we would expect to see equal numbers of the four types of offspring.

Since most offspring had a parental phenotype it can be concluded that the genes for body color and wing size are located on the same chromosome.

Linked genes tend to be inherited together because they are located near each other on the same chromosome.

Recombinants have different phenotypesthan the parents.

Morgan, who discovered that genes were located on chromosomes, proposed that some process must occasionallybreak the physical connection betweengenes on the same chromosome.

This is now called crossing over, which accounts for the recombination of linked genes.

Sturdevant, a student of Morgan’s, devised a way to construct genetic maps, an ordered list of the genetic loci along a particular chromosome.

He predicted that the further apart two genes are on a chromosome, the higher the probability that crossing over will occur.

Using recombination frequencies to construct a genetic map

1 map unit is equal to 1% recombination frequency

a second crossoverwould cancel out thefirst and reduce the observed b-vgrecomb. frequency

A genetic map based on recombination frequencies is called a linkage map and is based on the assumption that the probability of a crossover between two genetic loci is based on the distance separating the loci.

The distances between genes are expressed as map units, with one map unit equivalent to a 1% recombination frequency.

Partial Genetic Map of a Drosophila Chromosome The numbers represent the

distance between that locus and the locus for aristae length.

Investigators performed crosses that indicated bar-eye and garnet-eye alleles are 13 map units apart, scallop-wing and bar-eye alleles are 6 units apart, and garnet-eye and scallop-wing alleles are 7 units apart. What is the order of these alleles on the chromosome?

Given the following recombination frequencies, what is the correct orderof the genes on the chromosome?A-B, 8 map units; A-C, 28 map units;A-D, 25 map units; B-C, 20 map units;B-D, 33 map unitsa)A-B-C-D d) B-A-C-Db)D-C-A-B e) D-A-B-Cc)A-D-C-B

Suppose alleles M and n are linked on one chromosome, and m and N are linked on the homologous chromosome. Individuals homozygous for M and n are mated with individuals homozygous for m and N. Their offspring are crossed with homozygous recessive individuals, and the following results are recorded:Mn/mn 232 mN/mn 240MN/mn 15 mn/mn 13

How many units apart are these genes on the chromosome?

The frequency of crossing over between linked genes A and B is 35 percent; betweenB and C, 10 percent; between C and D, 15percent; between C and A, 25 percent; between D and B, 25 percent. The sequence of the genes on the chromosome is:

a. ACDB b. ACDBc. ABDC d. ABCDe. ADCB

Some Chromosomal Systems of Sex Determination

Males have 1 X

Sex Chromosome present in ovum

Unfertilized males; fertilized females

Although female mammals, including humans, inherit two X chromosomes, one X chromosome in each cell becomes almost completely inactivated during embryonic development. The inactive X in each cell of a female condenses into a compact object called a Barr body, which lies along the inside of the nuclear envelope.

If a female is heterozygous for asex-linked trait, about half her cellswill express one allele, while the others will express the alternate allele.

X inactivation and the Tortoiseshell Cat

Calico CatThe white areas are determined by yet another gene.

Meiotic Nondisjunction

Translocation

Alterations of Chromosome Structure

Down Syndrome/Trisomy 21

Klinefelter SyndromeMale with at least 1 extra X chromosome

XYY Karyotype or Jacob Syndrome

Testing a Fetus for Genetic Disorders