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Heredity: AKA Inheritance 17.0 Analyze the relationships within living systems 17.2 Examine the molecular basis of heredity and resulting genetic diversity. Inheritance: the way genes are passed down. • If a father doesn’t have a leg does that mean his children will be born with out a leg???? Hundreds of years ago people knew that somehow certain traits would be passed on to their children. Gregor Mendel was the first to predict the outcome of inheritance. The study of inheritance is Genetics. • Mendel did several different experiments on pea plant. • This is an example of wrinkled and smooth seeds. Genes and Alleles • A gene is a specific piece of DNA. • You have two genes that code for the same thing (ex. Height). Each of those two genes is called an allele. • So if something had the genes Aa one allele would be “A” the other would be “a”). Why two alleles for every gene? • Because organisms get 2 sets of chromosomes ……….one from the mother and one from the father. Phenotype • Is the physical characteristics of the trait. • In most cases it is what we can “see”. • Example: Freckles, dimples, attached earlobes etc. Genotype • Is the actual alleles that make up the trait. • In most all cases this is represented by 2 letters. • Example: AA, Aa, aa Dominant • A dominant trait is a trait that covers or hides the other trait of an allele. • For example, dark colors in hair usually overshadow the other allele. • In this class Dominant will always be represented with a capital letter. • Example: Say “D” is the allele for black hair and “d” is the allele for blond. The if an animal had Dd then it would have black hair. Recessive • A recessive trait is the trait that is hidden by the dominant trait. • In an animal with Dd for hair color the recessive allele is “d”. • The animal carries a gene for blond hair but you can’t tell by looking at it. Incomplete Dominance • Sometime traits do not have one clear dominant gene or one clear recessive gene • In incomplete dominance, traits appear to blend together • When crossing a red snapdragon with a white snapdragon, the offspring is pink if incomplete dominance occurs • The heterozygous genotype in incomplete dominance appears different than either of the homozygous alternatives – Example: red – pink - white Heterozygous • This is the term given two different alleles. • For example, Aa is heterozygous. • It may contain one gene for brown eyes(A) from the mother and one gene for blue(a) eyes from the father. Homozygous • This is the term given to the two same alleles. • For example, AA or aa • A person may contain 2 genes for -brown eyes – one from each parent (AA) or 2 for blue (aa). Can an Organisms Genotype or Phenotype be predicted. • Using genotypic and phenotypic ratios the probability of a specific genotype or phenotype appearing can be determined. • A Punnet Square is used to determine these ratios. Punnett Squares A a Parents alleles are: AA and aa a Aa a Aa aa aa Phenotypic and Genotypic Ratios • Phenotypic Ratios – Shows ratio of what is seen • Example: previous Punnett square: the phenotypic ratio is 2:2 • This means that the appearance of the offspring will have a 50% chance to be of the Dominant trait A, and 50% chance to be of the Recessive trait a. Phenotypic and Genotypic Ratios • Genotypic Ratios – Shows the potential genetic make-up of the offspring. • Example: previous Punnett square: the genotype ratio is 0:2:2 • Meaning the offspring will not be homozygous AA, 50% chance of being Heterozygous Aa, and 50% chance of being homozygous aa. Example B B b Bb Bb b Bb Bb • Angus cattle: coat color B is dominant b is recessive cross a BB cow with a bb bull Example • What is the Phenotypic ratio? • What is the Genotypic Ratio? Example • What is the Phenotypic ratio? – 4:0 B - black • What is the Genotypic Ratio? – 0:4:0 Heterozygous Bb - Black Example cont. • Bull - BbPp BP Bp BBPp Bp BBPp Bp BBpp bP BbPp bp Bbpp • Polled: • P is dominant p is recessive • BBpp BbPp Cow - BBpp Bbpp Cross a BB cow to a Bb bull. But also, another allele is considered. • Cross the Bb heterozygous polled bull with the BB homozygous recessive polled cow • All of the offspring have a black phenotype only Example cont. BBPp – black polled BBpp – black horned BbPp – black polled Bbpp – black horned Phenotypic Ratio 4:0 - black 2:2 - polled Genotypic Ratio 2:2:0 - Black 0:2:2 - Polled