Mendelian Genetics

Chapter 6 (6.3 – 6.5)

Section 6.3

Slide 3 of 25

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.

Slide 4 of 25

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

Slide 5 of 25

Flowers?

Slide 6 of 25

Slide 7 of 25

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

Slide 9 of 25

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

Slide 10 of 25

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

Slide 11 of 25

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

Slide 13 of 25

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

Slide 14 of 25

Punnett Square

Slide 15 of 25

So…

The three possibilities actually stand for something:AA = Homozygous Dominant

Homo = SameAa = Heterozygous

Hetero = Different aa = Homozygous Recessive

So this means that:A = Dominant allelea = Recessive allele

Slide 16 of 25

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 allelesAllele – 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 characteristicBB = Homozygote Dominant Bb = Heterozygote (Note: no dominant or recessive)bb = Homozygote Recessive

Slide 17 of 25

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

Slide 18 of 25

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

Slide 19 of 25

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?

Slide 20 of 25

1:2:1 = Homo dominant : Hetero : Homo recessive

3:1 = Dominant Phenotype : Recessive Phenotype

Slide 21 of 25

0:4:0 = Homo dominant : Hetero : Homo recessive

4:0 = Dominant Phenotype : Recessive Phenotype

Slide 22 of 25

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”

Slide 23 of 25

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)

Slide 24 of 25

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

Slide 25 of 25

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