Mendelian Genetics:


| Ch. 14 - Mendel and the Gene Idea | | Ch. 15 - The Chromosomal Basis of Inheritance


Ch. 14 - Mendel and the Gene Idea


Must Know:
- P, F1, F2, dominant, recessive, homozygous, heterozygous, phenotypic, genotypic
- genetics problems
- allele/gene
- pedigree

P (parental) Generation:
- true-breeding parents in a genetic cross

F1 (first filial) Generation:
- offspring of parental generation

F2 (second filial) Generation:
- offspring of F1 generation

1. alternative versions of genes cause variations in inherited characteristics (A & a)
2. one allele for each character from each parent
3. if two alleles are different, dominant will be fully expressed, recessive will have no noticeable effect
4. two alleles for each character separate during gamete production

Allele:
- alternative versions of genes that are the result of slightly different DNA sequences

Law of Segregation:
- if the parent has two different alleles, the offspring has a 50% chance of getting either allele

Law of Independent Assortment:
- each pair of alleles will separate independently during gamete formation

external image Offspring-of-monohybrid-cross-in-ratio.jpg
Homozygous:
- two same alleles
- dominant (RR)
- recessive (rr)

Heterozygous:
- two different alleles (Rr)

Phenotype:
- expressed physical trait, ex. purple

Genotype:
- genetic makeup, ex. Rr, rr

Testcross:
- used to determine if homozygous/heterozygous
- RR x rr = Rr, while Rr x rr = 1/2 Rr, 1/2 rr

Monohybrid Cross:
- cross involving one characteristic

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Dihybrid Cross:
- cross involving two characteristics

Complete Dominance:
- heterozygous and homozygous dominant are indistinguishable
- dominant allele is completely dominant

Codominance:
- two alleles are dominant and affect phenotype in two different but equal ways
- ex. red and white spotted flower

Incomplete Dominance:
- phenotype of offspring is between two parents
- ex. pink flower

Multiple Alleles:
- gene has more than two alleles
- ex. blood type

Pleiotropy:
- gene has multiple phenotypic effects
- ex. sickle-cell disease

Epistasis:
- gene at one locus alters effects of gene at another locus
- ex. golden retrievers

Polygenic Inheritance:
- two or more genes have additive effect on single character
- ex. height, skin colour
- fewer individuals at extremes, most in middle

external image pedigree.jpg
Pedigree:
- diagram that shows relationship between parents and offspring across 2 or more generations
- can be used to determine genomes of individuals and future offspring

Recessively Inherited Disorders:
- require two copies of defective gene in order to be expressed
- ex. cystic fibrosis, tay-sachs, sickle-cell

Lethal Dominant Alleles:
- require only one copy to be expressed
- ex. Huntington's disease


Ch. 15 - The Chromosomal Basis of Inheritance


Must Know:
- chromosome theory of inheritance, physical movement of chromosomes in meiosis, Mendel's laws of inheritance
- sex-linked genes
- alteration of chromosome number, structurally altered chromosomes, genetic disorders

Chromosome Theory of Inheritance:
- genes have specific locations on chromosomes, chromosomes segregate and assort independently

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Sex-Linked Gene:
- gene located on sex chromosome (X or Y)
- each egg contains an X chromosome, sperm 50-50 X or Y
- fathers pass sex linked genes to daughters but not sons, pass Y chromosome to son instead
- females express sex-linked trait like any other, but since males have only one X, that one allele is expressed
- majority genes on X chromosomes are not actually related to sex

X-Inactivation:
- regulates gene dosage in females
- one X in each cell becomes inactivated by methylation
- males and females have same dose of genes
- inactive chromosome condenses into Barr body, lies along inside of nuclear envelope

Linked Genes:
- located on same chromosome, tend to be inherited together during cell division

Genetic Recombination:
- production of offspring with new combination of genes inherited from parents

Recombinant:
- phenotype different from either parent

Parental Type:
- phenotype the same as one parent

Linkage Map:
- genetic map based on percentage of cross-over events

Map Unit:
- 1% recombination frequency
- express relative distances along chromosome


Alterations of Chromosome Number or Structure:

Nondisjunction:
- when homologous pairs or sister chromatids don't separate properly during meiosis

Aneuploidy:
- one gamete receives two copies of chromosome, one receives none
- offspring has incorrect chromosome number

Trisomic:
- fertilized egg receives three copies of chromosome

Monosomic:
- fertilized egg receives one copy of chromosome

Polyploidy:
- more than two complete sets of chromosomes
- 3n, 4n
- rare in animals, frequent in plants

Deletion:
- chromosomal fragment is lost
- chromosome is missing genes

Duplication:
- chromosomal fragment that broke off becomes attached to its sister chromatid
- zygote gets double dose of genes

Inversion:
- fragment breaks off and reattaches in original position but backward

Translocation:
- deleted chromosome fragment joins nonhomologous chromosome