MCB 10 Topic 8 & 9: Principles of Inheritance & Pedigrees

Chromosomal theory of inheritance

theory proposing that chromosomes are the vehicles of genes and that their behavior during meiosis is the physical basis of the inheritance patterns that Mendel observed

2 Mendelian Principles of Inheritance

1. Law of Segregation

2. Law of Independent Assortment

Law of Segregation

each gamete only gets one allele (random)

-predicting a single gene and phenotypes (feature)

-think Punnett square

Law of Independent Assortment

the alleles of two (or more) different genes (& different chromosomes) get sorted into gametes independently of one another

-the random configuration of homologs in Metaphase I is the basis of independent assortment

-predict the frequency for segregation of alleles of two different genes in a cross

-if you cross two heterozygotes the phenotypes segregation will be 3:1

-if you cross two individuals that are heterozygotes for 2 different genes the phenotypic segregation will be 9:3:3:1

gene linkage

genes controlling 2 different traits are located near each other on the same chromosome

-only genes that are located on different chromosomes have independent assortment during meiosis

-caveat: crossing over

Modes of Inheritance

-autosomal recessive

-X-linked recessive

-autosomal dominant

-X-linked dominant

-Y-linked

-mitochondrial inheritance (mother --> child)

Strategies for Pedigrees

easier to eliminate modes of inheritance than determine the specific mode of inheritance due to the small number of progeny

autosomal recessive

-disease phenotype in individuals with 2 recessive copies

autosomal recessive pedigree characteristics

-both parents look normal (both carriers)

-both sexes equally effected

-about 1/4 of offspring affected when both parents are carriers (large families)

-if both are affected parents, 100% of children will be effected

autosomal dominant

disease occurs in heterozygotes or those homozygotes for the dominant gene

autosomal dominant pedigree characteristics

-1 parent affected

-1/2 children affected when 1 parent is affected

-both sexes equally effected

-tends not to skip a generation

sex linkage

-usually refers to the X-chromosomes because the Y chromosome is very small and has very few genes. Also, genes on the Y chromosome tend to be linked to male fertility, so often times, they can't get passed on

-women have 2 X-chromosomes and men have an X and Y

-male can only contribute the X that he has

X-linked recessive

occurs in all males with mutant allele and females who are homozygous for the mutant allele

X-linked recessive pedigree characteristics

-often appear to skip a generation (man passes down to daughter, but its recessive so she's just a carrier

-male offspring are not carriers; possess the mutant

-majority are effected are male

X-linked dominant

occurs in all males with a mutant allele and females who are heterozygous or homozygous for the mutant

X-linked dominant pedigree characteristics

-2x females effected than males

-affected males only have effected daughters

-affected daughters have both sons and daughters that are effected

mendelian inheritance

genetic inheritance in diploid sexually reproducing organisms considering a single gene trait

;

unlinked

/

two different homologs

modes of inheritance (def.)

-rules that explain single-gene traits and inheritance over generations

-used to predict and counsel families with genetic conditions