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Simple Genetics

Simple Genetics. Exploring Unlinked Genes to Sex-Linked Genes. Overview. Autosomal Genes Punnett squares Dihybrid Crosses Bigger Punnett squares... Sex-Linked Genes Thank your mom. The Allele . Any of the alternative versions of a gene that may produce distinguishable phenotypic effects.

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Simple Genetics

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  1. Simple Genetics Exploring Unlinked Genes to Sex-Linked Genes

  2. Overview • Autosomal Genes • Punnett squares • Dihybrid Crosses • Bigger Punnett squares... • Sex-Linked Genes • Thank your mom

  3. The Allele Any of the alternative versions of a gene that may produce distinguishable phenotypic effects. • Phenotype: The observable physical and physiological traits of an organism, which are determined by it’s genetic makeup. • Genotype: The genetic makeup, or set of alleles, of an organism. Alleles are found on chromosomes • One per chromosome • Separated in meiosis • Maternal vs. Paternal chromosomes • Dominant vs. Recessive • Codominance vs. incomplete dominance

  4. H-words • Homozygous: Having two identical alleles for a given trait • Dominant vs. Recessive • Heterozygous: Having two different alleles for a given trait

  5. Mendell (He’s the pea dude) • Independent Assortment: Each pair of alleles assort independently of each other during gamete formation • Law of Segregation: Two allele pairs segregate into different gametes

  6. Biology’s favorite shape The Punnett Square: A diagram used in the study of inheritance to show the predicted genotypic results of random fertilization in genetic crosses between individuals of a known genotype • Shows genotypes of possible offspring of two parents

  7. Monohybrid Crosses A cross where only one trait is being explored • Based on when a single set of chromosomes is separated Diagramed by a 2 x 2 Punnett Square • Define alleles • Set up square • Cross Sample: A heterozygous green pea plant is crossed with a yellow pea plant. Show the cross. A a A = green a = yellow a a Aa aa Aa aa

  8. Sample Problems • Two pea plants heterozygous for pod color are crossed. Define allele notation and diagram a Punnett Square for this cross. • A homozygous dominant red flower is crossed with a homozygous recessive white flower. They produce all pink flowers, what is the reason for this? What would be produced if a pink offspring was crossed with another pink offspring? Diagram a Punnett Square. • A blue eyed man and brown eyed women have four children, none of which have blue eyes. Does blue eyes appear to be dominant or recessive?

  9. Dihybrid Crosses A cross where two genes are explored simultaneously • Physically based on alleles located on two sets of chromosomes, which then segregate into gametes. • Genes must be unlinked (not on the same chromosome) • 9:3:3:1 Requires a 4 x 4 Punnett Square • Determine possible gamete formations • Set up square • Cross! Remember this? It’s a dihybrid cross!

  10. 9:3:3:1, the magic number Refers to the phenotypic ratio expected from a normal dihybrid cross of two heterozygous parents. This was devised by Mendell. 9: the expected number of offspring with all dominant traits 3: the expected number of offspring with one dominant and one recessive trait 3: the reciprocal of the first ‘3’ 1: expected number of offspring with all recessive traits

  11. Sample Problem Cross: AaBb x AaBb Possible gametes (Parent 1): AB, Ab, aB, ab Possible gametes (Parent 2): AB, Ab, aB, ab AB Ab aB ab AB Ab aB ab

  12. Practice Problems • Two dogs heterozygous for brown fur and blue eyes are crossed. Diagram this cross. What is the probability of the dogs having a white fur-green eyed dog? • A dog true-breeding for brown fur and blue eyes is crossed with a dog true-breeding for white fur and green eyes. Diagram the cross. What is the probability that these parents will have a white fur-green eyed offspring?

  13. Sex-Linkage • The miracle of the Y chromosome • Does not include alleles found on it’s homologous X chromosome • Males have increased susceptibility to these types of alleles • Two X chromosomes makes a female one X and one Y makes a male!

  14. Crosses for traits that are sex-linked Also known as x-linked traits • Notation: XA Xa and Y for males • X and Y represent the sex chromosome • When crossing, X X will always represent female and the X Y will always represent male. Carrier: An individual who is heterozygous for a specific trait, thusly it affects their offspring, but not their phenotype.

  15. Sample Problem! Hemophilia, a sex linked disorder, causes difficulty in blood clotting, it is a recessive disorder. A hemophiliac man marries a woman whose father was also a hemophiliac, but she is healthy. What is the probability that this couple will have a hemophiliac daughter? • Determine genotypes of parents • Remember, this is a monohybrid cross, set up a square • Preform the cross (do not forget correct notation) XHXh Father: XhY Mother: XHXh 1/2 Chance of hemophilic daughter D: Xh Y XHXhXhXh XHY XhY XH= healthy Xh= hemophiliac

  16. A Last Problem... • A healthy man and a healthy women have a hemophilic child. The man believes this is evidence that his wife has had an affair. What is the genotype of the man and his wife? Is this evidence of an affair?

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