Chapter 8: Mendelian Genetics

The Law of Independent Assortment and Dihybrid Crosses:

The Law of Independent Assortment: states that alleles at each locus segregate independently of other alleles at other loci during metaphase I of meiosis

• Mendel observed independent assortment and unlinked genes in his pea plants, dihybrid cross, RY, Ry, rY, ry with a ratio of 9:3:3:1

Rules of Probability:

Both occur: multiplication rule P(A and B) = P(A) * P(B)

Either occur: addition rule P(A or B) = P(A) + P(B)

Pedigree Analysis:

pedigree analysis: the way geneticists determine an individual’s genotype

• rare, dominant, autosomal: all affected individuals will have a parent that is also affected. if a parent is homozygous dominant, all of the parent’s offspring will be affected. if parent is heterozygous, half of the offspring will be affected
  • ex: huntington’s disease
• rare, recessive, autosomal: two unaffected parents can have an affected offspring
  • ex: albinism

Extensions on Mendelian Genetics:

The Law of Segregation: states that when an organism makes gametes, each gamete receives just one gene copy, which is selected randomly.

This law makes 2 assumptions:

1. there are only 2 alleles for each gene
2. there is complete dominance between those two alleles

Multiple Alleles (assumption 1):

• most genes are polymorphic (have more than 2 alleles)
  • ex: rabbit coloration and human blood type

Codominance, Incomplete Dominance, Hybrid Vigor (assumption 2):

• Complete dominance: occurs when a heterozygote fully expresses one allele, masking the other
• Codominance: both alleles at a locus affect the allele (striped flowers, AB blood type)
• Incomplete dominance: phenotype of heterozygotes is intermediate (pink flowers)
• Quantitative traits: complex traits, continuous, controlled by many different genes
  • ex: height, weight, IQ, age at sexual maturity
• Hybrid vigor: occurs when different true-breeding homozygotes cross, producing hybrid offspring with stronger, larger phenotypes

Epistasis and Environmental Influence:

Epistasis: occurs when an allele at one locus affects the phenotypic expression of an allele at a second locus

• ex: dog fur color, human skin color

Environmental Influence: there is usually an interaction between an organism’s genotype and the environment it lives in

• Penetrance: relative percentage of individuals with a specific genotype that show the phenotypic trait
  • 1000 plants, 800 purple = partial penetrance of purple is 80%
• Expressivity: the extent to which an individual with the phenotype shows the trait
  • 800 purple plants, range between deep purple and light purple: deep purple have high expressivity, light purple have low expressivity
• Heritability: measure of the relative contribution of genetic factors (as opposed to environmental factors) that produce a variation of a character in a population

Autosomal Linkage:

• Law of Independent Assortment does not hold true when two genes are on the same chromosome

• Genetic linkage: genes residing on the same chromosome tend to be inherited with each other

  Three points:

  1. independent assortment of chromosomes maximizes genetic variation
  2. linkage between genes reduces genetic variation
  3. crossing over between linked genes restores some of the genetic variation lost because the genes are linked

Crossing Over and Recombination:

recombination = # of recombinant offspring / total # of offspring

Gene mapping: purpose is to locate where genes are on an individual chromosome

Sex Linkage:

• Sex linkage: occurs when a gene resides on a sex-chromosome (X) so phenotypic expression of the trait that the gene controls depends on the gender of the individual

• Males are hemizygous: only one copy of sex-linked genes

• Y chromosome is much smaller than X, so it is logical there are more genes on X than Y

  • X-linked traits ex: red-green color blindness, hemophilia, muscular dystrophy

Characteristics of X-linked recessive phenotypes:

• they are more common in males than in females
• affected males can only pass the mutation on to daughter
• daughters who will receive only one recessive allele are heterozygous carriers
• the mutant phenotype can skip generations

REVIEW:

• autosomal dominant: if two affected parents have an unaffected offspring, the trait must be dominant
• autosomal recessive: if two unaffected parents have an affected offspring the trait must be recessive
• x-linked dominant: affected fathers pass the traits on to all of their daughters, all affected sons have affected mother
• x-linked recessive: more males affected than females, all daughters of affected fathers are carriers

Bacterial Inheritance:

Bacterial conjugation: process of genetic exchange among bacteria through direct contact

• bacterial cells connect with one another through a sex pilus, cytoplasmic bridge called a conjugation tube forms, genetic material can pass from one cell to another
• type of horizontal gene transfer: movement of genes from individual to individual without sexual reproduction
• binary fission: clones, horizontal transfer: increases genetic diversity and allows bacteria to survive in an unstable environment
• plasmids: small circular DNA molecules, play important role in the transfer of genes between bacterial cells, antibiotic resistance, conjugation, breakdown of hydrocarbons. replicate independently and insert themselves into recipient’s cell genome
• transformation: occurs when a bacterial cell picks up naked DNA from environment, horizontal gene transfer

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