Sex Chromosomes

So far, what do you know about sex chromosomes?

In addition to their role in determining sex, the sex chromosomes, especially X chromosomes, have genes for many characters unrelated to sex.

We call these sex-linked alleles.

Female cells can differ from male cells in two ways:

1. Female cells show dark spots of chromatin (called Barr Bodies) during interphase, male cells do not.

2. Female cells contain 2 X chromosomes and males contain only one X chromosome.

The Y chromosome carries few genes.

There are very few genes on the Y chromosome that are common on the X chromosome, and because of that, little crossing over may occur between an X and a Y.

eg.) Calico cats

Male cats tend to be black (XBY) or orange (X0Y). Female cats can be black (XBXB), orange (X0X0) or calico (XBX0) – a mixture between black and orange.

Very few male cats can be calico, why?

Those who do, carry a hidden X chromosome, and are likely sterile.

• A male embryo does not differ from a female fetus until the 6th/7th week of pregnancy.

• At this point, the “testes determining factor” (TDF) gene on the Y chromosome is activated.

• The TDF gene initiates the production of a protein that stimulates the testes to begin secreting male hormones.

Examples of sex linked traits.

a. Hemophilia - lack or deformity of blood clotting factor VII or IX.

b. Red Green colorblindness

c. Pattern baldness - sex influenced not sex-linked.

i. Humans carry two alleles for baldness.

ii. In females the allele for baldness is recessive but in males, due to testosterone, it is dominant.

We can also perform monohybrid crosses between sex chromosomes.

For example: Brown eye color (B) is dominant to blue (b).Eye color is carried on the X chromosome.

Homozygous dominant female XBXB (brown)Heterozygous female XBXb (brown)Homozygous recessive female XbXb (blue)Dominant male XBY (brown)Recessive male XbY (blue)

Draw a Punnett square for a cross between a heterozygous female with a recessive male. Calculate the phenotypic & genotypic ratios.

XBXb XbXb

XBY XbY

XB Xb

Xb

Y

In the F1 generation:

Phenotypic ratios:

1 brown eyed girl: 1 brown eyed boy: 1 blue eyed girl: 1 blue eyed boy

Genotypic ratios:

1XBXb: 1XbXb: 1XBY: 1XbY

Example #2Is it possible to get a blue eyed female from crossing a blue eyed female with a brown eyed male? Explain.

XBXb XBXb

XbY XbY

Xb Xb

XB

Y

No it is not possible, all females would be browned eyed

Chromosomal TheoryThe chromosomal theory is as follows:i) Chromosomes carry genes, the units of hereditary

ii) Paired chromosomes segregate during meiosis. Each sex cell or gamete has half the number of chromosomes found in a somatic cell

iii) Chromosomes sort independently during meiosis. Each gamete receives one of the pairs and that one chromosome has no influence on the movement of a member of another pair

iv) Each chromosome contains many different genes

Chromosome Mapping and Gene Linkage• A single chromosome contains many genes linked together and so does the other chromosome in the homologous pair.• The sequence of genes on each chromosome pair should match each other exactly.• Gene linkage reduces the chance for genetic recombination and variety among the offspring. • Parts of a chromosome holding many genes, may separate and switch places with the matching part of the other chromosome = crossing over.

• The closer genes are to each other, the less likely they will separate during crossing over = linked genes.• Scientists use crossover frequencies on genes to determine their positions on chromosomes

eg.) if the crossover frequency of a gene is 5%, then the two genes are 5 map units apart.

• Crossover frequency is determined by the following formula:

crossover % = number of recombinations x 100 total number of offspring• Gene markers are usually recessive genes that are easily observed in offspring and can be used to identify other genes found on the same chromosome.

• By using crossover frequencies, we can determine gene maps.• Gene maps show the relative positions of genes on a chromosome (loci).• Gene maps are constructed by:

- ordering fragments of DNA

- studying chromosomal alterations

- performing crosses to see how frequently crossing over occurs between fragments.

Problem 1: 3 genes A, B, C AB – 12% CB – 7% AC – 5%

A B C

12 map units

7 map units

5 map units

Problem 2: AB - 3% BC - 28% AC - 31%

A B C31 map units

28 map units

3 map units

Problem 3:Genes X Y Z

X - 10 15 Y 10 - 5Z 15 5 -

XY

Z15 map units

10 map units 5 map

units

Crossover Frequency of Some Genes on Chromosome #6

Genes Cross-over FrequencyDiabetes(1) and Ovarian cancer (2) 21%

Diabetes (1) and RH blood group(3) 12%

Ragweed allergy (4) and RH blood group(3) 10.5

RH blood group(3) and ovarian cancer (2) 9%

Ragweed allergy (4) and ovarian cancer (2) 19.5

Hint: Start here

Transposons (see handout)

Gene Therapy: when defective genes are replaced with normal genes in order to cure genetic diseases

Human Genome Project: to determine the complete sequence of the 3 billion DNA subunits (bases), identify all human genes, and make them accessible for further biological study.