Mendelian Genetics

Genes and The Environment The product of a genotype is generally not a rigidly defined phenotype, but a range of phenotypic possibilities, the norm of reaction, that are determined by the environment. In some cases the norm of reaction has no breadth (for example, blood type). Norms of reactions are broadest for polygenic characters. For these multifactorial characters, the environment contributes to their quantitative nature.

Sun (smaller) and shade leaves in a hosta

Young Arnold Schwarzenegger

Environment and intelligence tests

Environmental influence – flower color in hydrangea – Blue color occurs in more acid soil

Phenotype depends on the interaction of environment and genes – even identical twins accumulate phenotypic differences

Mendelian Inheritance in Human Pedigree Analysis

Key to reading Genetic pedigrees

Is a widow’s peak a dominant or recessive trait? (a) Key Male Female Affected male Affected female Mating Offspring 1st generation Ff Ff ff Ff 1st generation Ww ww ww Ww 2nd generation 2nd generation FF or Ff ff ff Ff Ff ff Ww ww ww Ww Ww ww 3rd generation 3rd generation ff FF or Ff WW or Ww ww Figure 14.15 Pedigree analysis. Widow’s peak No widow’s peak Attached earlobe Free earlobe Is a widow’s peak a dominant or recessive trait? (a) Is an attached earlobe a dominant or recessive trait? b)

Sex and Inheritance Sex-linked genes are carried on the sex chromosomes – like hemophilia and color blindness that are carried on the X and thus show up more often in males or hairy ears that is carried on the Y and only shows up in males Sex-influenced genes are carried on regular autosomal chromosomes but the expression is influenced by the sex of the individual - The sex influence appears to be related to levels of male sex hormones – in particular testosterone – more testosterone leads to greater expression of the trait

Sex-linked - hemophilia

Sex-linked - hemophilia

Queen Victoria and hemophilia

Pattern baldness – sex-influenced Method of inheritance Genotype Phenotype Male Female BB Bald Bb Not bald bb

Pattern baldness in the Adams Family

Albinism Parents Normal Aa Normal Aa Sperm A a Eggs Aa Normal (carrier) AA Normal A Figure 14.16 Albinism: a recessive trait. Aa Normal (carrier) aa Albino a

Cystic Fibrosis

Cystic fibrosis One in 25 whites of European ancestry is a carrier, 1 in 2500 is affected. The normal allele codes for a membrane protein that transports Cl- between cells and the environment. If these channels are defective or absent, there are abnormally high extracellular levels of chloride that causes the mucus coats of certain cells to become thicker and stickier than normal. This mucus build-up in the pancreas, lungs, digestive tract, and elsewhere favors bacterial infections. Without treatment, affected children die before five, but with treatment can live past their late 20’s or even longer.

Sickle-cell anemia It affects one of 400 African Americans, 1 in 12 African Americans carries the trait. It is caused by the substitution of a single amino acid in hemoglobin. When oxygen levels in the blood of an affected individual are low, sickle-cell hemoglobin crystallizes into long rods. This deforms red blood cells into a sickle shape.

Sickle-cell anemia – pleiotropy

Distribution of Sickle-Cell Anemia

Distribution of malaria

Achondroplasia – a dominant trait Effects 1 in 10,000 people, 99.99% of population are homozygous recessive for trait

Huntington’s disease – lethal autosomal dominant

Huntington’s Disease The dominant lethal allele has no obvious phenotypic effect until an individual is about 35 to 45 years old. The deterioration of the nervous system is irreversible and inevitably fatal. Any child born to a parent who has the allele for Huntington’s disease has a 50% chance of inheriting the disease and the disorder. Recently, molecular geneticists have used pedigree analysis of affected families to track down the Huntington’s allele to a locus near the tip of chromosome 4.

Huntington’s Disease Sufferer Woody Guthrie Huntington’s Disease Sufferer

Genetic Counseling Consider a hypothetical couple, John and Carol, who are planning to have their first child. In both of their families’ histories cystic fybrosis, a recessive lethal disorder is present, and both John and Carol had brothers who died of the disease.

Genetic Counseling While neither John and Carol nor their parents have the disease, their parents must have been carriers (Aa x Aa). John and Carol each have a 2/3 chance of being carriers and a 1/3 chance of being homozygous dominant. The probability that their first child will have the disease = 2/3 (chance that John is a carrier) x 2/3 (chance that Carol is a carrier) x 1/4 (chance that the offspring of two carriers is homozygous recessive) = 1/9.

Genetic Counseling If their first child is born with the disease, we know that John and Carol’s genotype must be Aa and they both are carriers. The chance that their next child will also have the disease is 1/4. Mendel’s laws are simply the rules of probability applied to heredity.

Biochemical and genetic tests (a) Amniocentesis (b) Chorionic villus sampling (CVS) 1 Ultrasound monitor Ultrasound monitor Amniotic fluid withdrawn Fetus 1 Placenta Fetus Suction tube inserted through cervix Chorionic villi Placenta Cervix Uterus Cervix Uterus Centrifugation Fluid Several hours Several hours Biochemical and genetic tests 2 Fetal cells Several weeks Fetal cells Figure 14.19 Testing a fetus for genetic disorders. 2 Several weeks Several hours 3 Karyotyping

George Arlene Sandra Tom Sam Wilma Ann Michael Carla Daniel Alan Tina Figure 14.UN07 George Arlene Sandra Tom Sam Wilma Ann Michael Carla Figure 14.UN07 Daniel Alan Tina Christopher