Complex Inheritance Patterns

Sex Determination, Sex-Linked Traits, & Sex Determination, Sex-Linked Traits, &

Sex Influenced TraitsSex Influenced Traits

April 17, 2008April 17, 2008

Gender is determined by a pair of chromosomes called the sex chromosomes.two types: X and Y – not always homologous

females have two X chromosomesmales have one X and one Y chromosome

In humans, the remaining 22 pairs of chromosomes are homologous and are referred to as autosomes.

X chromosome is larger than Y ‘X’ contains 900-1400 genes ‘Y’ contains fewer than 100

genes – mostly related to “maleness”

In females, one of the X chromosomes is inactivated. X-inactivation occurs randomly

in each body cell. Form of dosage compensation. Inactivated X-chromosome

condenses into a darkly staining region called a Barr body.

Example: Calico color in cats

Calico cat

Traits controlled by genes on the sex chromosomes are called sex-linked traits. Mostly X-linked traits; very few Y-linked traits. Males disproportionately affected by recessive alleles on the X

chromosome. Examples:

Red-green color blindness Hemophilia

Traits located on the autosomes can sometimes be affected by the proportion of sex hormones produced by the body. Ultimately sex hormone production is prescribed by the sex

chromosomes. These are called sex-influenced traits Example:

Male pattern baldness

OtherColorblindn

ess Tests

A recessive X-linked trait Affects 8-12% of males in the United States; less than 0.5% of

females. Results when the color-detecting cones in the retina of the eyes

function ‘poorly’ in discriminating between red and green colors.

Normal vision

Red-green colorblind vision

PROBLEM: A man with normal vision and a

woman who is heterozygous for the colorblind allele want to have a child. ‘B’ is the allele for normal vision ‘b’ is the allele for color

blindness

What is the probability that their child will be colorblind?

Could they have a colorblind daughter?

XXBB YYXB XBXB XBYXb XBXb XbY

SOLUTION: 25% No, they cannot have a colorblind daughter. The only child that

could be affected would be a boy.

HemophiliaA recessive X-linked disorderResults in a delayed clotting of bloodWill it be more prevalent in males or females?

Why?A man with hemophilia has children with a

woman who is a carrier for hemophilia. What is the chance that their next child will have hemophilia? Their next son? Their next daughter?

Male pattern baldness is a sex-influenced trait. The gene, B1, for baldness is dominant in males, but recessive in females. The gene, B2, produces normal hair growth in both men and women. The baldness gene is also an autosomal trait.

If a man with male pattern baldness and genotype B1B2 marries a woman who is not bald with genotype B2B2, what are the chances that they will have a son with male pattern baldness? (HINT: This requires a dihybrid cross.)

XB1 XB2 YB1 YB2

XB2 XXB1

B2

XXB2

B2

XYB1B2

XYB2B2

XB2 XXB1

B2

XXB2

B2

XYB1B2

XYB2B2

XB2 XXB1

B2

XXB2

B2

XYB1B2

XYB2B2

XB2 XXB1

B2

XXB2

B2

XYB1B2

XYB2B2

50% female (none bald); 25% male (balding); 25% male (not balding)

Did the balding sons get their baldness gene from their mother or their father?

ADAM Inc., Male Pattern Baldness. 14 Apr 2008. U.S. National Library of Medicine 16 Apr 2008 <http://www.nlm.nih.gov/medlineplus/ency/imagepages/17083.htm>

ADAM Inc., Various Tests for Color Blindness. 2007. New York Times Company. 16 Apr 2008 <http://www.nytimes.com/imagepages/2007/08/01/health/adam/9962Colorblindnesstests.html>.

Biggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., 2007.

Color in Computer Graphics. 25 Feb 1998. Cornell University Program in Computer Graphics. 16 Apr 2008 <www.graphics.cornell.edu/online/tutorial/color/>.

Waggoner, Terrace L.. "About Color Blindness (Color Vision Deficiency)." Colors for the Color Blind. U.S. Naval Hospital, Pensacola, FL. 16 Apr 2008 <http://www.toledo-bend.com/colorblind/aboutCB.html>.

"X chromosome." Genetics Home Reference: Your Guide to Understanding Genetic Conditions. 14 Apr 2008. U.S. National Library of Medicine. 16 Apr 2008 <http://ghr.nlm.nih.gov/chromosome=X>.

“X chromosome.” Photo Researchers, Inc. 16 Apr 2008.