Theoretical Genetics Gregor Mendel

Objectives 4.3.1 – Define dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, genotype, phenotype, carrier, and test cross. 4.3.2 – Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid.

Review of DNA Structure Eukaryotic DNA is divided among several chromosomes (humans have 23 pairs of chromosomes – one set from each parent). Chromosomes are divided into units called genes. Genes code for the body’s proteins: eye pigment. Genes come in versions called alleles Eye color gene – either brown or blue allele. Result of mutation.

Definitions Locus The particular position of a gene on homologous chromosomes. Homologous: codes for the same things.

Definitions Homozygous vs. heterozygous alleles An organism with two identical alleles for a character is homozygous for that character. Organisms with 2 different alleles for a character are heterozygous. Bb BB bb

Definitions Dominant & recessive allele (complete dominance) A dominant allele is strong and is expressed (or seen) in the phenotype. (Abbreviated with capitals) A recessive allele (if present in a heterozygous individual) is not expressed; it is hidden because it is weak. Reces- sive alleles only show up if the in- dividual is homo- zygous. (Lowercase letters) Bb BB bb

Definitions Incompletely dominant alleles Sometimes, neither allele (when heterozygous) can overpower the other. They produce a blend. = allele for white flower color (Incomplete dominance) = allele for red flower color (Incomplete dominance) Pink Red White flowers flowers flowers

Definitions Codominant alleles Pairs of alleles that both affect the phenotype when present in a heterozygote. Ex: Blood groups: A, B, O are alleles that code for a sig- nal protein on the membrane surface. O is recessive, but A & B codominant. Punnet square

Definitions Genotype vs. phenotype: All the many alleles of an organism (for eye color, hair color, seed appearance, etc.) make up its genotype [genetic make-up; think: type of genes]. An organism’s physical characteristics make a phenotype.

The science of genetics Genetics - the study of heredity. Heredity – a characteristic of life: the passing of traits from parents to offspring. Traits - characteristics such as eye color, body size, sickle cell anemia, etc. The science of genetics be- gan with an Austrian monk named Gregor Mendel in the 1860s, working with pea plants (Pisum sativum).

The science of genetics Mendel studied pea flower, seed & pod color, and seed shape. Did careful pollination work & counted offspring. Ex: he mated plants with white flowers to plants with purple flowers, or plants with wrinkled seeds to plants with smooth seeds (the parental generation, or P).

The science of genetics Mendel’s work with peas: Mendel found that plants of the first generation (F1) were often identical. Ex: all had purple flowers or all had yellow seeds. →→ But mating the first generation (F1) plants gave a 3:1 ratio in the 2nd generation (F2). *F stands for filial (son) F1 generation F1 generation

Mendel’s Law of Segregation A pair of allelic genes for a particular character, like eye color, separate (segregate) in equal ratios during gamete formation. During meiosis, alleles are separated on a 1:1 ratio into sperm and eggs Ex: if the genotype = Gg, half the sperm must contain G and the other half must contain g.

Punnet grids To make a Punnet grid: Determine the geno- type of the parents. Make every possible combination of gamete. Combine the sperm and the egg. Determine the phenotypes from the genotypes of the offspring. A monohybrid cross: one locus is considered Male parent Female parent Sperm Eggs P represents the gene that makes the purple flower pigment. The allele p is mutated, and the protein is defective. All F1 offspring are Pp and purple.

Punnet grids To make a Punnet grid: F1 generation To make a Punnet grid: Determine the geno- type of the parents. Make every possible combination of gamete. Combine the sperm and the egg. Determine the phenotypes from the genotypes of the offspring. F2 generation

Punnet grids Incomplete dominance Ex: red and white snapdragon flowers Blending of color A 1:2:1 ratio of offspring

Test cross To test an unknown individual: Testing a suspected heterozygote by crossing it with a known homozygous recessive individual. 1 of 2 possible outcomes shows the genotype of the unknown parent.

Objectives 4.3.3 – State that some genes have more than two alleles (multiple alleles). 4.3.4 – Describe ABO blood groups as an example of codominance and multiple alleles.

Mendel’s Law of Independent Assortment Alleles of genes on nonhomologous chromosomes assort independently during meiosis. All blonds do not have blue eyes. Hair color & eye color are on different chromosomes. Chromosomes are shuffled.

Punnet grids To make a dihybrid cross (crossing 2 pairs of genes): From parents’ genotypes, determine gametes. every possible combination Make every possible combination of gametes. (independent assortment)

Punnet grids To make a dihybrid cross: From parents’ genotypes, determine gametes. Combine the sperm and eggs, and determine phenotypes.

Multiple alleles Codominant alleles Pairs of alleles that both affect the phenotype when present in a heterozygote. Ex: Blood groups: A, B, O are alleles that code for a sig- nal protein on the membrane surface. O is recessive, but A & B codominant. Punnet square

Multiple alleles Some genes have more than two alleles. ABO blood group shows codominance of multiple alleles (IA and IB are codominant; i is recessive). O (ii) is the universal donor; AB accepts any blood.

Punnet square Punnet square for blood groups I is dominant, but there are 2 types: IA & IB i is recessive. Here, a mother with blood group O mates with a father who’s AB.

Punnet square Punnet square for sex-linked traits For genes found on a sex chromosome, X or Y. Remember, boys have only one X chromosome, so they are more likely to get these diseases, like color-blindness or hemophilia. Notice, 50% of babies are boys, and 50% are girls. Girls get 2 X’s, so they are likely to have a good back-up copy and don’t get affected.

Multiple alleles Polygenic inheritance Two alleles on each of three genes have an additive effect on skin color.

Genetic diseases Sickle-cell anemia The gene for hemoglobin is mutated. An individual with two copies of the recessive allele can’t move oxygen around the body well.

Genetic diseases Sickle-cell anemia A carrier is an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele. Carrier genotype = Hhs - resistant to malaria The gene for hemo- globin (H) is mutated. Both Hhs HH hshs Hhs Homozygous recessive individuals often die young.

Sex-linkage and Pedigrees

Diseases linked to sex chromosomes Sex-linked traits: genes are on the sex chromosomes XX individuals are female XY individuals are male. Theoretically 50:50 male:female. Actually ~51:49 male:female at birth then 50:50 by age 3 (boys weaker).

Diseases linked to sex chromosomes Color-blindness is caused by a defective gene for a pig- ment receptor in the eye. Gene is on X chromosome. Boys only get 1 X, so they are more likely than girls to get this problem.

Diseases linked to sex chromosomes Color-blindness is caused by a defective gene for a pig- ment receptor in the eye. Gene is on X chromosome. Boys only get 1 X, so they are more likely than girls to get this problem. Retina of the eye

Diseases linked to sex chromosomes Hemophilia: a genetic disease in which the body does not produce sufficient amounts of a clotting factor. As a result, fibrin (necessary to maintain the blood clot) does not form, so the individual is more likely to bleed long- er from a wound, and to bleed internally. The gene for this clotting factor is on the X chromo- some. Boys have no back-up copy of the X.

Genetic diseases - sex linkage Hemophilia: genotypic & phenotypic ratios The female carrier is heterozygous* *Note: the daughters can be either heterozygous for sex-linked diseases or homozygous if a carrier mother marries a diseased man. 1 in 10,000 males is affected with hemophilia, and 1 in 100,000,000 females.

Genetic diseases Hemophilia: A case study Alexei, tsarevich of Russia inherited the disease from his mother. The original mutation in this lineage occurred in Queen Victoria.

Pedigrees X-linked dominant trait passed from the father: Only his daughters get the disease.

Pedigrees Y-linked trait (carried on the Y chromosome). Only boys get the disease.

Karyotype A karyotype is a picture of the body’s chromosomes It shows abnormalities, also the individual’s sex.

Karyotype A karyotype is a picture of the body’s chromosomes It shows abnormalities, also the individual’s sex. Down Syndrome – three copies of chromosome 21.