Chapter 11.2Sex Determination, Sex-Linked Traits, &

Sex Influenced Traits

Complex Inheritance Patterns

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.

Sex Determination

Sex-Linked Traits 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

Red-Green Color BlindnessOther

Colorblindness Tests

Variations in Red-Green Color Blindness

Red-Green Colorblindness 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

Color blindness Problem: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?

XB YXB 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?

Sex-Linked Practice Problem:

Sex-Influenced Traits – A Challenge ProblemMale 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.)

Challenge Problem SolutionXB1 XB2 YB1 YB2

XB2 XXB1

B2XXB2

B2XYB1B

2XYB2B

2

XB2 XXB1

B2XXB2

B2XYB1B

2XYB2B

2

XB2 XXB1

B2XXB2

B2XYB1B

2XYB2B

2

XB2 XXB1

B2XXB2

B2XYB1B

2XYB2B

250% 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.

References

Chapter 11.3Karyotypes and Nondisjunction

Chromosomes and Human Heredity

Some inherited traits can be identified at the chromosome level.

Geneticists use karyotypes.Chromosomes are stained.A photomicrograph is taken of a cell’s

chromosomes during metaphase.Chromosomes pairs are arranged in order of

decreasing size.

Karyotypes

Example Karyotype

Fetal Tests

Test Benefit Risk

Amniocentesis• Diagnosis of chromosome abnormalities• Diagnosis of other defects

• Discomfort for expectant mother

• Slight risk of infection• Risk of miscarriage

Chorionic villus sampling

• Diagnosis of chromosome abnormality• Diagnosis of certain genetic defects

•Risk of miscarriage•Risk of infection•Risk of newborn limb defects

Fetal blood sampling

• Diagnosis of genetic or chromosome abnormality

• Checks for fetal blood problems and oxygen levels

• Medications can be given to the fetus before birth

• Risk of bleeding from sample site

• Risk of infection• Amniotic fluid might leak• Risk of fetal death

Amniocentesis procedure

NondisjunctionCell division in which either the homologous

pairs or sister chromatids do not separate correctly, resulting in gametes with an abnormal number of chromosomes.

Types of NondisjunctionMonosomy – having only one of a particular type

of chromosome.Trisomy – having a set of three chromosomes of

one kinde.g. Down syndrome (Trisomy 21)

Results in distinctive facial features, short stature, heart defects, and mental disability

Nondisjunction in Sex Chromosomes Females can survive with only one X chromosome or with

three. Males can survive with either an extra X or an extra Y. Males cannot survive with only a Y chromosome.

Genotype XO XXX XXY XYY OY

Phenotype

Female with Turner’s

syndrome (sterile)

Nearly normal female

Male with Klinefelter’s syndrome (sterile)

Normal or nearly normal

maleResults in

death

Biggs, Haggins, Holliday. Biology (Glencoe Science)(Teacher Wraparound Edition). New York: GLENCOE MCGRAW HILL, 2007.

Farabee, M. J. Human Genetics. 2001. 01 Apr. 2009 <http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookhumgen.html>.

Downschild. Digital image. About Downs Syndrome. 2008. The Coventry and Rugby Down's Syndrome Support Group. 01 Apr. 2009 <http://www.downssupport.org.uk/aboutdowns.html>.

"Genetics and pregnancy loss." Miscarriage Management™ -. 2009. Sydney IVF Limited. 01 Apr. 2009 <http://www.miscarriage.com.au/basepage.cfm?id=16>.

Karyotype. Digital image. AP Biology. 01 Apr. 2009 <http://www.nkellogg.com/apbiology.htm>.Nondisjunction. Digital image. 01 Apr. 2009

<http://porpax.bio.miami.edu/~cmallery/150/mendel/heredity.htm>.

References