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Mendel’s Laws of Heredity Why we look the way we look... What is genetics? Genetics is the study of heredity, how traits are passed from parent to offspring The father of Genetics Gregor Mendel • His most famous experiment was done with pea plants. • Was an Austrian Monk that lived in the mid 1800s Mendel observed that the height of pea plants varied Mendel crossed different size plants to see if he could predict the height of the offspring Mendel’s cross between tall pea plants yielded all tall pea plants. His cross between small pea plants yielded all small pea plants. X = X = Mendels’ cross between tall pea plants and small pea plants yielded all tall pea plants. x = Mendel then crossed these second generation tall pea plants and ended up with 1 out 4 being small. x = Lets Start heredity These are my parents, who do I look like? My dad? My Mom? What is heredity? The passing on of characteristics (traits) from parents to offspring Genetics is the study of heredity Mendel used peas... They reproduce sexually They have two distinct, male and female, sex cells Their traits are easy to isolate Mendel crossed them Fertilization - the uniting of male and female gametes Cross - combining gametes from parents with different traits Questions What What What What What What did Mendel cross? are traits? are gametes? is fertilization? is heredity? is genetics? Rule of Unit Factors Each organism has two alleles for each trait –Alleles - different forms of the same gene –Genes - located on chromosomes, they control how an organism develops Rule of Dominance The trait that is observed in the offspring is the dominant trait (uppercase) The trait that disappears in the offspring is the recessive trait (lowercase) Law of Segregation A parent randomly passes only one allele for each trait to each offspring Questions... How many alleles are there for each trait? What is an allele? How many alleles does a parent pass on to each offspring for each trait Questions... What do we call the trait that is observed? What case (upper or lower) is it written in? What about the one that disappears? What case is it written in? Phenotype & Genotype Phenotype - the way an organism looks – red hair or brown hair genotype - the gene combination of an organism – AA or Aa or aa Heterozygous & Homozygous Heterozygous - if the two alleles for a trait are different (Aa) Homozygous - if the two alleles for a trait are the same (AA or aa) Monohybrid Monohybrid Cross crossing parents who differ in only one trait (AA with aa) Questions... What is the phenotype? What is the genotype? What is homozygous? What is heterozygous? What is monohybrid crossing? Let’s Practice –Take a look at the genetics behind getting a widows peak Dominant and Recessive Genes This is lily Widows Peak A dominant gene will always Munster mask a recessive gene. A “widows peak” is dominant, not having a widows peak is recessive. If one parent contributes a gene for a widows peak, and the other parent doesn’t, the offspring will have a widows peak. Genetics All organisms have two copies of each gene, one contributed by the father, the other contributed by the mother. Homozygous - Two copies of the same gene Heterozygous - Two different genes Genetics For the widows peak: WW - has a widows peak Ww - has a widows peak ww - no widows peak Homozygous dominant Heterozygous Homozygous recessive Genetics Since Herman has no widows peak, he must be “ww”, since Lilly has a widows peak she could be either “WW” or “Ww” Definitely ww Either Ww or WW Homozygous recessive Heterozygous Homozygous dominant Genetics We can use a “Punnet Square” to determine what pairs of genes Lilly has • A Punnet Square begins with a box 2 x 2 Assume Lilly is heterozygous Ww Assume Herman is homoozygous recessive ww • One gene is called an “allele” W w w Ww ww w Ww ww • One parents pair is split into alleles on top, the other along the side • Each allele is crossed with the other allele to predict the traits of the offspring Genetics Notice that when Lilly is crossed with Herman, we would predict that half the offspring would be “Ww”, the other half would be “ww” Half “Ww”, Heterozygous, and will have a widows peak Half “ww”, Homozygous, and will not have a widows peak W w w Ww ww w Ww ww Genetics Another possibility is that Lilly might be “WW”, homozygous dominant. Assume Lilly is homozygous dominant WW W Assume Herman is homoozygous ww W w Ww Ww w Ww Ww Notice that all the offspring are heterozygous and will have a widows peak