A recessive trait is expressed when an individual holds TWO recessive alleles (homozygous recessive) Example s recessive genetic disorders include: Cystic fibrosis Albinism Tay-Sachs disease Galactosemia Individuals heterozygous for a recessive disorder will express the dominant trait (no disorder) but will be a carrier of the disorder
Some disorders are caused by dominant genes Those people without the disorder are homozygous recessive Examples of Dominant Genetic Disorders: Huntingtons Disease Achondroplasia Huntingtons Disease - http://www.youtube.com/watch?v=65xf1olEpQM http://www.youtube.com/watch?v=65xf1olEpQM
A diagram that traces the inheritance of a particular trait through several generations Read about Pedigrees on pp. 299-301 Complete the Mini-Lab on p. 300
Chapter 11.2 Sex Determination, Sex-Linked Traits, & Sex Influenced Traits
RED Flower x WHITE Flower ---> PINK Flower With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.
There are foxes that roam the forests in northern Baltimore County. They come in three colors, blue, gold, and green. This trait is controlled by a single gene with incomplete dominance. A homozygous (BB) individual is blue, a homozygous (bb) individual is gold, and a heterozygous (Bb) individual is green. What would be the genotypes and phenotypes of the offspring if a blue fox were crossed with a gold one?
red x white ---> red & white spottedCODOMINANCE With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.
Longhorns may be white (C W C W ), red (C R C R ) or roan (C R C W ). Roan longhorns have a mixture of both white hairs and red hairs due to a codominant gene. "A single copy can be expressed by just a few white hairs on the face or extremities, unevenly roaned patches, or an even mix of white and colored hairs all over the body. Two copies produce an almost white animal, with some pigment around the ears. What would the phenotypes and genotypes of the offspring if a red male were crossed with a white female?
Sickle cell disease is caused by the allele that controls the formation of the protein hemoglobin. Hemoglobin is the part of the red blood cell that carries oxygen. The allele for normal hemoglobin (A) results in red blood cells that are disc- shaped. The sickle cell allele (S) changes the hemoglobin and results in red blood cells that are sickle -shaped. People who are heterozygous for these two alleles (AS) have both normal and sickle-shaped red blood cells. PROBLEM: What is the risk that two people heterozygous for the sickle-shaped allele will have a child with sickle cell disease?
A single gene that has more than two possible alleles. MULTIPLE ALLELES ALLELE I A I B i CODES FOR Type "A" Blood Type "B" Blood Type "O" Blood Notice that that the allele for "O" (i) is recessive to the alleles for "A" & "B". With three alleles we have a higher number of possible combinations in creating a genotype. GENOTYPES I A I A I A i RESULTING PHENOTYPES Type A Type A IBIBIBiIBIBIBiType B IAIBIAIB Type AB iiType O
A woman with Type O blood (ii) and a man who is Type AB (I A I B ) have are expecting a child. What are the possible blood types of the child?
One allele hides the effects of another allele Primula flowers produce a blue pigment with the dominant Gene K However, the presence of a dominant Gene D will mask effect of Gene K (KKDD, KKDd, KkDD, KkDd will all appear another color)
Gender is determined by a pair of chromosomes called the sex chromosomes. two types: X and Y 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.
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
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? XBXBXBXBY XBXB XBXBXBXB XBYXBY XbXb XBXbXBXb XbYXbY SOLUTION: 25% No, they cannot have a colorblind daughter. The only child that could be affected would be a boy.
Other Colorblindness Tests
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?
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 gene expressed in the presence of testosterone
Male pattern baldness is a sex-influenced trait. The gene, B 1, for baldness is dominant in males, but recessive in females. The gene, B 2, 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 B 1 B 2 marries a woman who is not bald with genotype B 2 B 2, what are the chances that they will have a son with male pattern baldness? (HINT: This requires a dihybrid cross.)
XB 1 XB 2 YB 1 YB 2 XB 2 XXB 1 B 2 XXB 2 B 2 XYB 1 B 2 XYB 2 B 2 XB 2 XXB 1 B 2 XXB 2 B 2 XYB 1 B 2 XYB 2 B 2 XB 2 XXB 1 B 2 XXB 2 B 2 XYB 1 B 2 XYB 2 B 2 XB 2 XXB 1 B 2 XXB 2 B 2 XYB 1 B 2 XYB 2 B 2 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?
Traits affected by more than one gene Examples: Skin color Eye color Height
ADAM Inc., Male Pattern Baldness. 14 Apr 2008. U.S. National Library of Medicine 16 Apr 2008 ADAM Inc., Various Tests for Color Blindness. 2007. New York Times Company. 16 Apr 2008. 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. Waggoner, Terrace L.. "About Color Blindness (Color Vision Deficiency)." Colors for the Color Blind. U.S. Naval Hospital, Pensacola, FL. 16 Apr 2008. "X chromosome." Genetics Home Reference: Your Guide to Understanding Genetic Conditions. 14 Apr 2008. U.S. National Library of Medicine. 16 Apr 2008. X chromosome. Photo Researchers, Inc. 16 Apr 2008.
Chapter 11.3 Karyotypes and Nondisjunction
Some inherited traits can be identified at the chromosome level. Geneticists use karyotypes. Chromosomes are stained. A photomicrograph is taken of a cells chromosomes during metaphase. Chromosomes pairs are arranged in order of decreasing size.
TestBenefitRisk 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
Cell division in which either the homologous pairs or sister chromatids do not separate correctly, resulting in gametes