Complex Inheritance Patterns Chapter 11.2 Sex Determination, Sex-Linked Traits, & Sex Influenced Traits
Sex Determination • Gender is determined by a pair of chromosomes called the sex chromosomes. • two types: X and Y – not always homologous • females have two X chromosomes • males have one X and one Y chromosome • In humans, the remaining 22 pairs of chromosomes are homologous and are referred to as autosomes.
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 Blindness Other Colorblindness Tests
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. Red-green colorblind vision Normal 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? SOLUTION: • 25% • No, they cannot have a colorblind daughter. The only child that could be affected would be a boy.
Sex-Linked Practice Problem: Hemophilia • A recessive X-linked disorder • Results in a delayed clotting of blood • Will 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-Influenced Traits – A Challenge Problem 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.)
Challenge Problem Solution 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?
References 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.
Chromosomes and Human Heredity Chapter 11.3 Karyotypes and Nondisjunction
Karyotypes • 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.
Fetal Tests Amniocentesis procedure
Nondisjunction • Cell 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 Nondisjunction • Monosomy – having only one of a particular type of chromosome. • Trisomy – having a set of three chromosomes of one kind • e.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.
References 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>.