Mendelian Genetics Chapter 6 (6.3 – 6.5)

Section 6.3

Mendel Austrian Monk Did not know about genes, chromosomes, etc. Thought that traits were inherited as discrete units So he hypothesized that there were genes before anyone discovered them. He studied pea plants.

P, F1, & F2 Generations P – Parental Generation The original generation F1 – First filial generation The first generation produced by the controlled mating F2 – Second filial generation The second generation produced by the controlled mating

Flowers?

3 Important Conclusions 1. Traits are inherited as discrete units 2. Organisms inherit 2 copies of each gene, 1 from each parent 3. The 2 copies segregate during gamete formation Called the “Law of Segregation” Not that type of segregation The 2 copies separate during meiosis

Section 6.4

Chromosome Connection As it turns out, the alleles are located on chromosomes An allele is a version of a gene A gene is a sequence of nucleotides on a chromosome So alleles are different sequences of nucleotides in the same location

Homologous Chromosomes Genes have a specific location or locus on a chromosome Each parent contributes a chromosome to a homologous chromosome pair So each parent contributes an allele So 2 alleles, 1 from each parent

TO SUMMARIZE: A dominant allele is expressed as a phenotype when at least one allele is dominant. A recessive allele is expressed as a phenotype only when two copies are present. Dominant alleles are represented by uppercase letters; recessive alleles by lowercase letters. Both homozygous dominant and heterozygous genotypes yield a dominant phenotype

Section 6.5

Punnett Squares One parent at top of Square, other parent on left side Every Individual has 3 options: 1. Two CAPITALIZED (HH or GG) 2. Two lowercase (hh or gg) 3. One CAP, one lower (Hh or Gg) Then you combine the letter in the row with the letter in the column to fill in each box

Punnett Square

So… The three possibilities actually stand for something: AA = Homozygous Dominant Homo = Same Aa = Heterozygous Hetero = Different aa = Homozygous Recessive So this means that: A = Dominant allele a = Recessive allele

Genotypes Genotypes refer to the types of gene combinations that are possible for any characteristic (hair color) B = Brown Hair b = Blonde Hair  “B” & “b” are called alleles Allele – Different forms of the gene But the offspring gets 1 allele from mom & 1 allele from dad, so every zygote has 2 alleles for any characteristic 3 possible genotypes for any characteristic BB = Homozygote Dominant Bb = Heterozygote (Note: no dominant or recessive) bb = Homozygote Recessive

Expression The dominant allele will MASK the recessive allele, so that when both are present, the dominant phenotype is expressed, So Homozygous dominant (AA) codes for Dominant Phenotype Homozygous recessive (aa) codes for Recessive Phenotype Heterozygous (Aa) codes for Dominant Phenotype

Genotypes  Phenotypes Dominant and Recessive Phenotypes Appearance or character Eye color, seed color, seed shape, etc. There are ONLY 2 options Dominant Phenotype & Recessive Phenotype Brown or Blue eye color, Green or Yellow seed color, etc. Dominant phenotype is coded for by 2 genotypes Homozygous Dominant AND Heterozygous Recessive phenotype due to recessive genotype Homozygous Recessive

Answer the following 1. The gene for flower color has two traits: purple and white. Purple is dominant to white. We will use A for the Purple allele, and a for the white allele. A) What do we call AA? B) What color will AA be? C) What do we call Aa? D) What color will Aa be? E) What do we call aa? F) What color will aa be?

1:2:1 = Homo dominant : Hetero : Homo recessive 3:1 = Dominant Phenotype : Recessive Phenotype

0:4:0 = Homo dominant : Hetero : Homo recessive 4:0 = Dominant Phenotype : Recessive Phenotype

Probabilities Probability = # of Success Possibilities / Total Possibilities Coin: Probability of Heads = ½ On a coin, one side is heads and the other is tails 2 possibilities, but to get heads it is 1 of 2 Probability of rolling a 2 on a die = 1/6 On a die, there are 6 sides But only one of them is a “2”

Calculate probabilities Probability of rolling a “4” on a die? Probability of selecting a “red” card from a deck of cards? (Assume there are 13 cards in each suit, and there are 4 suits. 2 of them are “red” and 2 are “black”) Probability of getting a heterozygote offspring, when both parents are heterozygous? (Use a Punnett Square)

What about 2 traits? What if we want to calculate the probability of 2 things happening at the same time? ForEx: What is the probability of getting an offpsring to be AaBb when A is one gene and B is another gene? Calculate Probability of Aa AND Probability of Bb, then MULTIPLY THEM If parents are AAbb and aaBb, Probability of Aa = 1 & Probability of Bb = ½, MULTIPLY = 1/2

Finally The parents are GgHh & Gghh, what is the probability of GgHh? Using the Punnett Squares at the side, probability of Gg is 2/4 = ½ & probability of Hh = 2/4 = 1/2 MULTIPLY = ½ x ½ = ¼