Mendel's Genetics

Biology, Seventh Edition Solomon • Berg • Martin

Chapter 10

The Basic Principles The Basic Principles of Heredityof Heredity

Biology, Seventh Edition CHAPTER 10 The Basic Principles of Heredity

TerminologyTerminology

•Phenotype

•Genotype

•Locus

•Allele

•Dominant allele

•Recessive allele

•Homozygous

•Heterozygous

• Mendel’s principles of inheritance• Segregation

–During meiosis, alleles for each locus segregate

• Independent assortment–Alleles of different loci distributed randomly

into the gamete–Results in recombination–Production of new gene combinations not

found in parent

Seven characters in Mendel’s study of pea plants

Oneof Mendel’spea crosses

Genelociand theiralleles

• Monohybrid cross• Cross between homozygous parents• Differ at one locus

• Dihybrid cross• Differ at two loci

• Test cross• Between individual of unknown

genotype and homozygous recessive individual

Monohybridcross in guinea pigs

Dihybridcrossinguinea pigs

Test cross inguinea pigs

• Genetic ratios can be expressed as probabilities• Product rule predicts combined

probability of independent events• Sum rule predicts combined probability

of mutually exclusive events

Rulesof probability

• Segregation• Results from homologous chromosomes

separating during meiosis

• Independent assortment• Orientation of homologous chromosomes

on the metaphase plate determines how chromosomes are distributed

Chromosomalbasis forsegregation

• Linkage• Tendency for a group of genes on same

chromosome to be inherited together

• Recombination of linked genes• Results from crossing-over in meiotic

prophase I• By measuring frequency of

recombination, can construct linkage map of chromosome

Two-pointtest cross todetect linkagein fruit flies

Crossing-over

• Sex chromosomes• Cells of females of many species contain

two X chromosomes• Males have single X chromosome and

single, smaller Y chromosome• Y chromosome determines male sex in

most species of mammals

Sex determinationin mammals

X-linked red-green colorblindness

Dosage compensation in female mammals

• Incomplete dominance• Heterozygote is intermediate in

phenotype

• Codominance• Heterozygote simultaneously expresses

the phenotypes of both homozygotes

Incompletedominancein four o’clocks

Multiple alleles in rabbits

Geneinteractionin chickens

Epistasis in Labrador retrievers

• Norm of reaction• Range of phenotypic possibilities from a

single genotype under different environmental conditions

• Example is height in human–Can be modified by factors such as diet–Genes that affect height set norm of reaction–Environment molds phenotype within norm

of reaction

Polygenicinheritance inhuman skinpigmentation

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