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

- 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
- two same alleles
- dominant (RR)
- recessive (rr)

- two different alleles (Rr)

- expressed physical trait, ex. purple

- genetic makeup, ex. Rr, rr

- 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

- 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

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

- 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
- 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

- 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

- 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:

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

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

- fertilized egg receives three copies of chromosome

- fertilized egg receives one copy of chromosome

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

- chromosomal fragment is lost
- chromosome is missing genes

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

- fragment breaks off and reattaches in original position but backward

- deleted chromosome fragment joins nonhomologous chromosome