Patterns of Inheritance

Inheritance Hypotheses Blending Hypothesis – parental contributions combined Particulate Hypothesis – parents pass along discrete heritable units

Some Important Vocabulary Allele= an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. Homozygous = two identical alleles (Ex. tt, TT) Heterozygous = two different alleles (Ex. Tt) Genotype The two genes that an organism inherits for a certain trait (Example Tt, TT, or tt) Phenotype Physical characteristics (example: tall and short)

Analyzing Patterns Genetic Cross – controlled experiment to determine the inheritance of a trait P – parental generation F1 – first generation F2 – second generation P F1 F2

Homologous chromosomes Review Meiosis Of your father’s 46 chromosomes how many do you receive? Of your mother’s 46 chromosomes how many do you receive? So for each allele you share __(#) copy from mom and ___(#) copy from dad. If this is one of your chromosomes what two possible alleles would you have at this locus to pass on to your offspring? Y y Homologous chromosomes

General Patterns of Inheritance Alternative versions of genes cause variation Offspring inherit one copy from each parent Dominant alleles are expressed in phenotype Recessive alleles are expressed in phenotype in the absences of dominate alleles Alleles for genes separate during meiosis Gametes fuse randomly

Mendel’s Laws Father of modern genetics Researched pea plants Developed ideas of dominance and trait segregation Allelic Interactions Pleiotropy Epistasis Environment Polygenics

Mendel's Law of Segregation Each allele separates from the other so that the offspring get only one allele from each parent for a given trait. Let’s cross a heterozygous tall plant (Tt) with a short plant (tt). Each plant will give only one of its’ two genes to the offspring or F1 generation. T t Plant 1 (tall) Plant 2 (short) Possible zygotes after meiosis X

Law of Segregation and Punnett Squares Plant 1 (tall) Plant 2 (short) Possible zygotes after meiosis X Plant 2 Plant 1 Tt TT tt Tt T t t T

Developed ideas of dominance and trait segregation Allelic Interactions Complete dominance Incomplete dominance Co-dominance Pleiotropy Epistasis Environment Polygenics

Allelic Interactions Complete dominance: dominant allele fully expressed Incomplete dominance: neither allele fully expressed Co-dominance: both alleles fully expressed

Allelic Interactions: Complete Dominance The dominant allele is the only one seen in the phenotype Dominant vs. Recessive Gene Dominant Gene: A gene that always expresses itself. It is symbolized by a CAPITAL letter Recessive Gene: a gene that expresses itself only when a dominant form of the gene is NOT present. It is symbolized by a lower case letter Monohybrid cross is looking at 1 gene Dihybrid cross is looking at 2 genes Monohybrid Cross X

Allelic Interactions: Complete Dominance Practice Problem: Let's say that in seals, the gene for the length of the whiskers has two alleles.  The dominant allele (W) codes long whiskers & the recessive allele (w) codes for short whiskers. a)  What percentage of offspring would be expected to have short whiskers from the cross of two long-whiskered seals, one that is homozygous dominant and one that is heterozygous? b) If one parent seal is homozygous long-whiskered and the other is short-whiskered, what percent of offspring would have short whiskers? c) Is this a mono- or dihybrid cross?

Complete Dominance Dihybrid Cross What are the possible gametes from each of these pea plants? YyRr YyRr X Use FOIL if you want! == YR, yR, Yr, yr Y=yellow y=green R=round r=wrinkly

Allelic Interactions: Incomplete Dominance Neither allele fully expressed, they are mixed The phenotype of the heterozygote will be intermediate between the phenotypes of the two homozygotes Example: RR = Red snapdragon flower Rr = pink snapdragon flower rr = white snapdragon flower

Allelic Interactions: Incomplete Dominance Practice Problem: Cross a red snapdragon with a pink snapdragon flower. What will the genotype and phenotype % be for the offspring.

Co-Dominance If two alleles have a co dominant relationship, in the heterozygote both alleles will be completely expressed. Example: blood type

Example: Blood Types

Co-Dominance What would happen if you crossed a man with AB blood type with a women with O blood type? (a) Give the genotypes and phenotypes for the offspring. (b) How many of the offspring would have AO? _____% (c) How many of the offspring would have BO? _____% (d) How many of the offspring would have AB? _____% (e) How many of the offspring would have O? _____%

Sex Linked (X-Linked) Traits The genes that are located on the chromosomes are called sex-linked traits Many traits that are carried on the X chromosome do not have a corresponding spot on the Y chromosome. This causes for some unique possibilities for the offspring. Ex. Color blindness in humans, fruit fly eye color, hemophilia.

Analyzing Patterns Pedigree – illustration of relationships among family members over multiple generations

Inheritance and Genetic Disorders Hereditary disorders Autosomal or sex-linked Recessive or dominant Genetic disorders are often polygenic and influenced by environment

Autosomal Recessive Disorders Carriers – heterozygotes that have one allele but not the disorder Examples – Cystic Fibrosis, Tay Sachs

Autosomal Dominant Disorders Disorders that are expressed with only one copy of the allele Example – polydactyly Example – Huntington’s

Sex-linked Recessive Disorders Disorders that are on the sex chromosome Example – Hemophilia

Sex-linked Dominant Disorders

Pleiotropy Condition where a single gene influences multiple traits Example: Marfan’s Syndrome

Polygenics Traits that are determined by many genes Examples – eye color, skin color, height, etc.

Environment Phenotype = Genotype + Environment Epigenetics – study of changes in phenotype due to mechanisms other than changes in DNA sequence Epigenetics