Sex-linked Traits

REVIEW!!!!!What is it…Incompete Dominance or

Codominance????

• Zebra (black and white coat)

• White dog + black dog → gray dog

• Red flower + blue flower → purple flower

• Indian Corn

• REMINDER:• Mendel studied

autosomal gene traits, like hair texture

• Two copies of each autosomal gene affect phenotype. RR= Red flower– Autosomes are

chromosome pairs # 1-22

• Mendel’s rules of inheritance apply to autosomal genetic disorders as well.

• These are genetic disorders carried on chromosomes # 1-22

– A heterozygote for a recessive disorder is a carrier.

– Disorders caused by dominant alleles are uncommon.

(dominant)

Sex-linked TraitsSex-linked Traits• Traits (genes) located on the sex

chromosomes– 23 chromosome pair

• Sex chromosomes are X and Y• XX genotype for females• XY genotype for males• Many sex-linked traits carried on X

chromosome because Y chromosome contains very few genes and is mainly involved in sex determination

• Males and females can differ in there 23rd chromosomal pair (which are their sex chromosomes).• Genes on sex chromosomes are called sex-linked genes.

– Y chromosome genes in mammals are responsible for male characteristics.• 250 known genes

– X chromosome genes in mammals affect many traits.• 1100 known genes, including many that causeGenetic disorders

•Several methods help map human

chromosomes. • A karyotype is

a picture of all chromosomes in a cell.

X Y

Females can carry sex-linked genetic disorders.

• Males (XY) express all of their sex linked genes.• Expression of the disorder depends on which parent carries the

allele and the sex of the child.

XY

• Because Males have just one X chromosome, all X linked alleles are expressed in males, even if they are recessive.

• (normal vision male) x (Normal vision, carrier female) XCY x XCXc

• Remember: the Y chromosome in males does not carry traits.

• CC = normal• Cc = Normal, carrier• cc = colorblindness

XC

XC y

Xc

Sex-linked Trait Solution:Sex-linked Trait Solution:

XC XC

XC Xc

XC y

Xc y

¼ normal visionfemale

¼ normal vision, carrier female¼ normal male¼ colorblind male

XC

XC y

Xc

Example: Color Blindness

Various tests for color blindness.

Examples of Sex-Linked Recessive Disorders

• Red/Green Colorblindness – Difficulty perceiving differences between colors (red or green, blue or yellow).

• Hemophilia – Absence of one or more proteins necessary for normal blood clotting.

• Deafness• Cataracts – opacity in the lens that can lead to blindness• Night blindness – (Nyctalopia) rods do not work so that can not

see in the dark• Glaucoma – pressure in the eye that can lead to optic nerve

damage and blindness• Duchenne Muscular Dystrophy – progressive weakness and

degeneration of skeletal muscles that control movement due to absence of dystrophin (protein that maintains muscle integrity). Mainly in boys, onset 3-5 yrs, by 12 years can’t walk, and later needs respirator.