Sex-Linked Traits

• A human cell contains over 6 billion nucleotide pairs in one diploid cell. If that were put into letters the size in your textbook it would be over 4 million pages long.

• All this is neatly packaged into the 46 chromosomes in each diploid cell.

• We don’t completely know how to read these chromosomes, but we do know what some parts contain.

• Chromosomes #21 and #22 are the smallest autosomes. Chromosome 22 has about 43 million nitrogen bases, and chromosome 21 has about 32 million nitrogen bases.

• These two chromosomes were the first two chromosomes of which their sequences were determined.

• Chromosome 22 has up to 545 genes. Some are very important. Genetic disorders on chromosome 22 could cause alleles for leukemia, or neurofibromatosis (tumor causing disease of the nervous system).

• Chromosome 22 also has stretches of repeating sequences, in which rearrangements occur.

•The structure of chromosome 21 is similar. •Chromosome 21 has about 225 genes. •Mutations in those genes could cause alleles for amyotrophic lateral sclerosis (ALS), also called Lou Gehrig’s disease.

•Chromosome 21 also has long repeated sequences that don’t code for any proteins.

Pedigree analysis reveals Mendelian patterns in human inheritance

In these family trees, squares symbolize males and circles represent females. A horizontal line connecting a male and female (--) indicates a mating, with offspring listed below in their order of birth, from left to right. Shaded symbols stand for individuals with the trait being traced.

Sex Linked Traits

• Many sex-linked genes are found on the X chromosome.

• More than 100 sex-linked genetic disorders have been mapped to the X chromosome.

• The Y chromosome is much smaller, and so it only contains a few genes.

• So, since males have only one X chromosome, all their X-linked alleles will be expressed, regardless of whether or not they are dominant or recessive.

• But in females, since they have 2 X chromosomes, they will only show a recessive trait if they have 2 copies of it (one on each chromosome).

• That means recessive phenotypes are more common in males if the trait is sex-linked.

• Also, males only have one X chromosome, so if they have a daughter, she will have that allele from her father. That allele may then show up again if she has a son.

Mother Father

Meiosis

Fertilization

XX XY

X XX Y

XX Daughter

Pattern Baldness In Humans: A Sex Influenced Trait

Baldness is an autosomal trait and is apparently influenced by sex hormones after people reach 30 years of age or older.

In men the gene is dominant, while in women it is recessive. A man needs only one allele (XBY) for the baldness trait to be expressed, while a bald woman must be homozygous for the trait (XBXB).

What are the probabilities for the children for a bald man and woman with no history of baldness in the family?

Sex-Linked Disorders in Humans

Duchenne muscular dystrophy, affects about one out of every 3,500 males born in the United States. People with Duchenne muscular dystrophy rarely live past their early 20s. The disease is characterized by a progressive weakening of the muscles and loss of coordination. Researchers have traced the disorder to the absence of a key muscle protein called dystrophin and have tracked the gene for this protein to a specific locus on the X chromosome.

Posture changes during progression of Duchenne muscular dystrophy.

Color Blindness In Humans: An X-Linked Trait

Numbers That You Should See If You Are In One Of The FollowingFour Categories: [Some Letter Choices Show No Visible Numbers]

Sex-Linked Traits: 

Write down what you see on the screen.

A.

B .

C.

D.

Color Blindness In Humans: An X-Linked Trait

Numbers That You Should See If You Are In One Of The FollowingFour Categories: [Some Letter Choices Show No Visible Numbers]

Sex-Linked Traits: 

1. Normal Color Vision: A: 29,  B: 45,  C: --,  D: 26  

2. Red-Green Color-Blind: A: 70,  B: --,  C: 5,  D: --  

3. Red Color-blind: A: 70,  B: --,  C: 5,  D: 6  

4. Green Color-Blind: A: 70,  B: --,  C: 5,  D: 2

Hemophilia is a sex-linked recessive trait defined by the absence of one or more of the proteins required for blood clotting.