mendelian genetics chapter 6 (6.3 – 6.5). section 6.3
TRANSCRIPT
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.
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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
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Flowers?
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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
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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
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Punnett Square
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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
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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
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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
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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?
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1:2:1 = Homo dominant : Hetero : Homo recessive
3:1 = Dominant Phenotype : Recessive Phenotype
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0:4:0 = Homo dominant : Hetero : Homo recessive
4:0 = Dominant Phenotype : Recessive Phenotype
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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)
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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
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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