Chapter 14

Mendel and Heredity  Gregor Mendel – Austrian Munk  Worked with heredity in pea plants  Wanted to determine how characters and traits were passed from generation to generation  Advantages of pea plants:  Short generation  Large offspring production  “either or” traits  Self pollinating  True breeding

Mendel’s Cross Fertilization Experiments  Crossed 2 distinct versions  Created a hybrid  True breeding plants were P-generation  Offspring were the F 1  If F 1 self pollinated the result was the F 2

Monohybrid Cross  Four principles revealed:  Alternate versions of genes account for variations - alleles  For each character, organisms inherit 2 copies, one from each parent  Alleles may be dominant or recessive  Law of segregation

Punnett Square  Used as a predictive tool to show potential offspring  Alleles are combined to show possible genetic outcomes  Homozygous dominant  Heterozygous  Homozygous recessive

Test Cross  Used by current geneticists to determine genetype  Use of a homozygous recessive crossed with the unknown  P? x pp  If all purple – PP  If half purple, half white - Pp

Law of Independent Assortment  Do alleles tend to follow one another?  Mendel paired two traits together and crossed  AABB x aabb (F 1 )  The resulting heterozygotes were then allowed to self pollinate (F 2 )  Dihybrid cross  9:3:3:1

Probability Rules  Probability scale – 0-1  1 = event is certain to occur  0 = event is certain not to occur  Multiplication rule – used for two or more independent events (Ex. Chance of having 5 boys in a row)  ½ x ½x ½ x ½ x ½ = 1/32  Addition rule – used for mutually exclusive events (Ex. Male vs. female)  ¼ = ¼ = ½

Degrees of Dominance  Alleles can show different degrees of dominance/recessive  Incomplete dominance is when neither allele is completely dominant (blended)  Codominance is when two alleles affect the individual in separate, distinguishable ways

Relationship Between Genotype and Phenotype  Alleles can looked at on three phenotipic levels  Ex. Tay Sachs disease  Only children that inherit two recessive alleles have the disease- organismal level is recessive  Heterozygotes produce enough of enzyme to be normal – biochemical level is incomplete dominance  Heterozygotes also produce equal numbers of normal and dysfunctional enzymes – molecular level is codomiance

Multiple Alleles and Pleitropy  Some genes exist with multiple alleles  Blood type  I A, I B, I  Also codominance  Some genes can have multiple phenotypic effects  Responsible for many symptoms associated with many genetic disorders

Epistasis  Phenotypic expression of one gene is affected by another gene  Ex. Labs

Polygenic Inheritance  Quantitative characters are the result of polygenic inheritance  Multiple genes determining the resulting phenotype  Ex. Height, skin color  Alleles have a cumulative affect

Human Traits and Pedigree  Geneticists can not manipulate traits in humans  To analyze they use a pedigree  Using information from family history about a particular trait

Autosomal Recessive Disorders  To get the disorder individual must inherit both recessive alleles  Heterozygotes are called carriers  When recessive disorders are rare, highly unlikely carriers will meet and mate

Autosomal Dominant Disorders  Individual must inherit only one dominant allele to have the disorder  Achondroplasia  Huntington’s disease

Genetic Testing and Counseling  Most recessive individuals come from normal parents 9carriers)  Problems with knowing?  Insurance  Loss of jobs  Types of testing  Amniocentesis  Chorionic villus sampling

Genetic Screening and Testing