Mendelian Genetics

Do You Know These Words Heredity Passing on of characteristics from parent to offspring Genetics Part of biology that studies heredity Gene Piece of DNA that codes for a polypeptide (specify certain trait) Trait Characteristic that is inherited Homologous Chromosomes Chromosome of the same type (pairs) Locus Specific location of a gene on a chromosome Diploid 2 copies of each chromosome = 2n Haploid 1 copy of each chromosome = 1n

Gregor Mendel Austrian monk A mathematician aware of probability The “father” of genetics Worked with pea plants

Why Peas? Peas have many traits with 2 different forms that are easy to distinguish (ex: green or yellow seeds; tall or short plants) Could easily control mating of the pea plants through cross-pollination Pea plants are small, they grow quickly and easily, and they produce a lot of offspring.

Mendel’s Crosses In one cross, Mendel crossed tall pea plants with short pea plants All of the offspring were tall! He then crossed two of the offspring tall plants The next offspring were ¾ tall and ¼ short Mendel said that 2 factors control each trait (these are genes found on chromosomes) Each form of a gene is called an allele. The two alleles are found on homologous chromosomes.

Mendel’s Crosses Parent x Parent = P1 generation Offspring x Offspring = F1 generation Offspring = F2 generation

Mendel’s Law of Dominance Mendel called the observable trait (all tall plants) dominant and the trait that disappeared (short plants) recessive The allele for tall plants is dominant The allele for short plants is recessive If T = tall and t = short TT = tall plant, Tt = tall plant, tt = short plant

Mendel’s Theory of Segregation Each parent passes on (at random) to offspring only one allele for each trait! A plant that is Tt can pass either T or t in a gamete. Ex: Humans have ____ chromosomes. When eggs and sperm are made, only half of these chromosomes are passed…one of each kind from mom and one of each kind from dad That way a fertilized egg (and the baby) will have the normal # of chromosomes

Mendel’s Theory of Segregation Diploid cells have pairs of genes on pairs of homologous chromosomes. The 2 genes of each pair are separated during meiosis and end up in different gametes.

More vocabulary! Phenotype = individual’s observable traits Ex: (tall, short) Genotype = genes an individual carries Ex: TT, Tt Homozygous = has 2 of the same alleles Purebred, true-breeding TT, tt Heterozygous = has 2 different alleles Hybrid Tt

Monohybrid Cross Cross a single trait Cross 2 parents that are homozygous for different alleles of a single trait Saw purple allele is dominant to (masks) white allele Purple flower x White flower All purple flower plants 705 purple 224 white About a 3:1 ratio

Now lets investigate the heredity seen in Reebops!

Punnett Squares Can be used to show all possible offspring! Ex.: R = round seed, r = wrinkled seed Homozygous round x homozygous wrinkled What F1 offspring can be produced? What are the genotypes and phenotypes?

F2 Generation Now let’s cross two of the heterozygous offspring from the F1 generation.

Dihybrid Cross Cross two traits Start with 2 parents that are homozygous for different alleles for 2 traits Round Yellow Seed x Wrinkled Green Seed R= round Y = yellow r = wrinkled y = green

Punnett Square Example: RRYY x rryy

Mendel’s Theory of Independent Assortment Gene pairs for different traits are inherited independently of each other Ex: Genes for round seed shape are inherited independently of genes for yellow seed color! Note: We will see later that this does not always apply

Test Cross I have a guinea pig that is black. I know that the black allele (B) is dominant to the brown allele (b). What are the possible genotypes for my guinea pig? I want to breed guinea pigs, and I want to have pure-bred guinea pigs! How do I know if he is pure-bred?

Test Cross My guinea pig may be BB or Bb. Both would show the black phenotype! To see if a dominant phenotype is due to a homozygous dominant or a heterozygous phenotype, I can… Cross it with one that is homozygous recessive! B B B b b b