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Mendelian genetics.

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Presentation on theme: "Mendelian genetics."— Presentation transcript:

1 Mendelian genetics

2 Gregor Mendel: Hereditary Genetics
Three Principles: Principle of Dominance Some alleles are dominant, others are recessive Principle of Segregation Alleles are separate from one another; each gamete only carries a single copy of each gene Principle of Independent Assortment Genes for different traits segregate independently of one another during the formation of gametes (sperm/egg) Video clip: Alleles & Genes

3 Mendel and His Beloved Peas
Mendel and His Beloved Peas Garden peas are: Self-pollinating Easy to experimentally cross-pollinate Clear traits—pure purple, pure white, etc. Trait: any single feature of an organism

4 What Mendel Noticed:  Whoa! P Generation (“Parent”)
F1 Generation (“First Filial”) (Kids) “Short” trait disappears… Whoa! F2 Generation (“Second Filial”) (Grandkids) …then reappears!

5 Did You Catch That? Where did that short plant (trait) come from?

6 Recessive Traits “Hide” Behind Dominant Traits
Recessive Traits “Hide” Behind Dominant Traits Sexual Organisms have two of each trait (homologous pairs) One of each pair is passed on to offspring by each parent. (p265)

7 Recessive Traits “Hide” Behind Dominant Traits
Recessive Traits “Hide” Behind Dominant Traits Different forms of the same trait are called alleles PLANT HEIGHT What TRAIT is being studied in this experiment? “Tall” … and … What are the TWO ALLELES (forms) of the trait being studied? “Short”

8 Other Traits That Mendel Studied
ALLELES ALLELES

9 Try one What trait is being studied?
What’s the dominant allele of the trait? What’s the recessive allele of the trait?

10 WHY? Note the Percentages In P Gen: ____% Tall, ____% Short
50:50 WHY? 100:0 ~75:25 In P Gen: ____% Tall, ____% Short In F1 Gen: ____% Tall, ____% Short In F2 Gen: ____% Tall, ____% Short 50 50 100 ~75 ~25

11 Mendel proposed that “Factors” segregate (separate) from pairs to be passed on to offspring, one from each parent! (p ) (Sound familiar?)

12 (Sound familiar?) Homologous chromosomes split up in Anaphase I so that only one allele of each trait gets passed on in your gametes!

13 The pair of alleles you have (for any trait) is called your GENOTYPE
The pair of alleles you have (for any trait) is called your GENOTYPE Example Trait: Nose Shape (Let us pretend that round nose is dominant to pointy nose) (usually, letters are used to represent alleles, upper case for dominant) N n Alleles: ___ (dominant), ___ (recessive) Genotypes: ____, ____, ____ NN Nn nn “Homozygous Dominant” “Homozygous Recessive” “Heterozygous”

14 The pair of alleles you have (for any trait) is called your GENOTYPE
Again! NN Nn nn Genotypes: ____, ____, ____ This genotype is described as __________________________ This genotype is described as _____________ This genotype is described as __________________________ “Homozygous Dominant” (Purebred) “Heterozygous” (hybrid or carrier) “Homozygous Recessive” (Purebred)

15 How Did You Get Your Genotypes?
One from Mom One from Dad Each set of alleles is a homologous pair, one from Mom and one from Dad!

16 What is this person’s genotype?
This is called your PHENOTYPE (an expressed feature you can see!) N N ________________ Homozygous Dominant What kind of nose do they have? Round? Or Pointy?

17 What is this person’s genotype?
Remember: Dominant overshadows recessive! N n ________________ Heterozygous What is this person’s PHENOTYPE? Round? Or Pointy?

18 What is this person’s genotype?
Recessive alleles show only if no dominant is present! n n ________________ Homozygous Recessive What is this person’s phenotype? Round? Or Pointy?

19 Another Example: What’s the TRAIT? What are the ALLELES?
Which is dominant? PP pp Pp

20 Remember: Only one of each allele is passed on from each parent!
Pp Pp What is Mom’s genotype? What is Dad’s genotype? GERM CELL Heterozygous Ova (either) Heterozygous Pp GERM CELL Sperm (either)

21 Let’s Look At the Offspring Probabilities
Ova (either) What if this egg meets this sperm? What if this egg meets this sperm? What if this egg meets this sperm? Sperm (either) What if this egg meets this sperm? What if this egg meets this sperm?

22 This Is Called A Punnett Square
Punnett squares allow us to predict probabilities about offspring

23 What percentages does this Punnett square predict?
How many Purple? ______ 75% (3/4) How many White? ______ 25% 25% 25% (1/4) How many homozygous dominant? ___ 25% (1/4) How many heterozygous? ______ 25% 25% 50% (2/4) How many homozygous recessive? ___ 25% (1/4)

24 What is the probability that this couple will have a baby with the “a” disease?
Healthy allelle = A Disease allele = a 25% (1/4)

25 What is the probability that this couple will have a baby who is a CARRIER for the “a” disease?
Healthy allelle = A Disease allele = a 50% (2/4) A CARRIER is a heterozygote (he or she can “pass on” the trait but does not exhibit it)

26 Your Turn! (For disease “A”): What if a homozygous dominant man marries a carrier woman? AA Aa Offspring Genotype: - HD (AA) = ___% - Het (Aa)= ___% - HR (aa)= ___% ____ X ____ Dad Mom 50 A a 50 A AA Aa Offspring Phenotype: - Healthy* = ___% - Disease = ___% - *(Healthy) Carrier = _____% 100 A AA Aa 50

27 One More Time! (For disease “A”): What if a carrier man marries a woman with the disease? Aa aa ____ X ____ Offspring Genotype: - HD (AA) = ___% - Het (Aa)= ___% - HR (aa)= ___% Dad Mom a a 50 50 A Aa Aa Offspring Phenotype: - Healthy* = ___% - Disease = ___% - *(Healthy) Carrier = _____% 50 50 a aa aa 50


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