Mendelian genetics

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 Traits That Mendel Studied ALLELES ALLELES

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

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

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

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)

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”

 Mendel proposed that “Factors” segregate (separate) from pairs to be passed on to offspring, one from each parent! (p265-266) (Sound familiar?)

Homologous chromosomes split up in Anaphase II so that only one allele of each trait gets passed on in your gametes!

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) Alleles: ___ (dominant), ___ (recessive) Genotypes: ____, ____, ____ N n NN Nn nn “Homozygous Dominant” (Purebred) “Homozygous Dominant” (Purebred) “Heterozygous” (hybrid or carrier)

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

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) 

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?

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

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)

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

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) 

 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

 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