Patterns of Inheritance

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

Some Important Vocab 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 (same) Heterozygous = two different alleles 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 – F 1 – first generation – F 2 – second generation

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

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

Each gene (allele) separates from the other so that the offspring get only one gene from each parent for a given trait. Let’s cross a dominant tall plant (TT) with a short plant (tt). Each plant will give only one of its’ two genes to the offspring or F 1 generation. TT x tt TTtt Law of Segregation

Law of Independent Assortment Hypothesis: Inheritance of seed shape is independent of inheritance seed color. Medal crossed two genetic traits: – Yellow vs. Green – Round vs. Wrinkled Results: Yes, inheritance of seed shape is independent of seed color. Conclusion: Chromosomes assort independently during meiosis

Developed ideas of dominance and trait segregation Allelic Interactions – Complete dominance – Incomplete dominance – Codominance Pleiotropy Epistasis Environment Polygenics

Punnett Squares A square which can be used to show the random combinations of genes which are possible when a sperm fertilizes an egg. TtTtTt The genes from one parent go here. The genes from the other parent go here.

Allelic Interactions Complete dominance – dominant allele fully expressed Incomplete dominance – neither allele fully expressed Codominance – both alleles fully expressed

Allelic Interactions: Complete Dominance –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

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 pure long-whiskered and the other is short- whiskered, what percent of offspring would have short whiskers?

Allelic Interactions: Incomplete Dominance Neither allele fully expressed 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 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 plant with red flowers with a plant with blue flowers? (a) Give the genotypes and phenotypes for the offspring. (b) How many of the plants would have red flowers? _____% (c) How many of the plants would have purple flowers? _____ % (d) How many of the plants would have blue flowers? _____ %

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. Colorblindness in humans, fruit fly eye color, hemophilia.

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

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

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

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