Chapter 11 Human Genetic Analysis
1. Complex inheritance of traits does not follow inheritance patterns described by Mendel. 2. Many traits result from alleles with a range of dominance, rather than a strict dominant or recessive relationship. 3. Gene expression is often related to whether a gene is located on an autosome or on a sex chromosome.
Codominance! When two dominant alleles are expressed at the same time, both forms of the trait are displayed.! Different from incomplete dominance because both traits are displayed.
Example: red flower x white flower = red and white flowered offspring equal number of red and white flowers
Example: Black horse x white horse = roan coat equal number of black and white hairs
Intermediate Traits " Incomplete dominance - an individual displays a trait that is intermediate between the two parents. Example: red flower x white flower = pink flower " Neither the red nor the white allele is completely dominant
Multiple Alleles! Genes with three or more alleles are said to have multiple alleles.! When traits are controlled by genes with multiple alleles, an individual can have only two of the possible alleles for that gene. Example: Blood types in humans; hair color; eye color! Three different alleles I A, I B, and i result in four different blood phenotypes A, AB, B, and O.
X-Linked Traits " The trait is carried by females ONLY on the X chromosome through a recessive allele. " Males will HAVE the disorder because they only have one X chromosome. " The X chromosome has many genes, some of which cause genetic disorders.
Traits influenced by the Environment " An individual s phenotype often depends on conditions in the environment. Example: fur color in Siamese cats # Dark fur around cooler parts of the body- ears, nose, paws, and tails. Example: skin tone in humans # Exposure to the sun alters the color of the skin.
Mutations " Changes in genetic material. " The harmful effects produced by inherited mutations are called genetic disorders. " Many mutations are carried by recessive alleles in heterozygous individuals. carriers
Genetic Counseling o Genetic counseling is a form of medical guidance that informs people about genetic problems that could affect them or their offspring.
1. Genetic Abnormality " Rare, uncommon version of a trait. 2. Genetic Disorder " An inherited condition that sooner or later will cause mild to severe medical problems.
3. Syndrome " A recognized set of symptoms that characterize a given disorder. 4. Disease " Illness caused by infectious, dietary, or environmental factors, NOT by inheritance of mutant genes.
Disorder Mutation Chromosome Color blindness P X Cystic fibrosis P 7q Down syndrome C 21 Hemophilia P X P Point mutation, or any insertion/deletion entirely inside one gene D Deletion of a gene or genes C Whole chromosome extra, missing, or both T Trinucleotide repeat disorders: gene is extended in length Klinefelter syndrome C X Sickle-cell disease P 11p Tay Sachs disease P 15
2007 Paul Billiet ODWS PEDIGREE CHARTS
What is a pedigree chart? " a chart of the genetic history of a family over several generations. " used to study the transmission of hereditary conditions 2007 Paul Billiet ODWS
Constructing a Pedigree " Female " Male
Connecting Pedigree Symbols " Married Couple " Children
Symbols used in pedigree charts " Affected " X-linked.. " Carrier " Deceased 2007 Paul Billiet ODWS
Symbols used in pedigree charts " Normal male " Affected male " Normal female " Affected female 2007 Paul Billiet ODWS
Organizing the pedigree chart " A pedigree chart of a family showing 20 individuals 2007 Paul Billiet ODWS
Organizing the pedigree chart # Generations are identified by Roman numerals. I II III IV 2007 Paul Billiet ODWS
Organising the pedigree chart " Individuals in each generation are identified by Arabic numerals numbered from the left " Therefore the affected individuals are II3, IV2 and IV3 I II III IV 2007 Paul Billiet ODWS
Interpreting a Pedigree Chart 1. Determine if the pedigree chart shows an autosomal or X-linked disease.! If most of the males in the pedigree are affected and the carriers are female, THEN the disorder is X-linked.! If it is a 50/50 ratio between men and women, THEN the disorder is autosomal.
Example 1 of Pedigree Charts $ Is it Autosomal or X-linked?
Answer $ Autosomal
2. Determine whether the disorder is dominant or recessive.! If the disorder is dominant, one of the parents must have the disorder.! If the disorder is recessive, neither parent has to have the disorder because they can be heterozygous.
Example 2 of Pedigree Charts $ Dominant or Recessive?
Answer $ Dominant
Example 3 of Pedigree Charts " Dominant or Recessive?
Answer " Recessive
% Several genes can influence a trait- polygenic trait. % Determining the effect of these genes is difficult. Example: A horse with red hair mates with a horse with white hair, and their offspring has both red and white hair. How can this be?