Ch 4- Mendelian Genetics and Pedigrees

gene

an inherited factor that determines a characteristic

alleles

different variations of a gene

locus

the place on a chromosome where an allele is found

genotype

the set of alleles an organism has

phenotype

the manifestation or appearance of a characteristic

homozygous (homozygote)

a diploid organism with 2 identical alleles at given locus

heterozygous (heterozygote)

a diploid an organism with 2 different alleles at a given locus

complex diseases vs single gene diseases

• complex diseases: many genomic variants (polygenic) interacting with environmental factors

• single gene disease: one genomic variant involved (i.e. sickle cell anemia, caused by mutation in hemoglobin beta gene on chr 11)

Gregor Mendel

discovered the principles of inheritance using peas

What does random assortment result in?

results in genetic variation and the underlying basis of Mendel’s observations (dihybrid crosses)

random assortment

chromosomes are randomly distributed in Metaphase I

monohybrid cross

• uses pure breeding plants (homozygous)

• a cross between plants involving only one character

conclusions of monohybrid cross

• each F₁ plant must possess two genetic factors (alleles) encoding a character

• principle of segregation

• the 2 alleles separate with equal probability

• the concept of dominance

principle of segregation

the 2 alleles of a gene separate and each gamete only received

the concept of dominance

• the trait that remains unchanged in the F₁ generation is dominant

• the trait that disappears in the F₁ generation is recessive

multiplication rule

• the probability of 2 or more independent events occurring together is calculated by multiplying their independent probabilities

• “and” = multiply

addition rule

• the probability of any one of the 2 or more mutually exclusive events is calculated by adding the probabilities of these events

• “or” = add

dihybrid cross

• crosses with parents that differ in two characters

• reveals the principle of independent assortment

principle of independent assortment

• when 2 alleles separate, their separation is independent of the separation of other alleles at other loci

• traits on different chromosomes or loci are unliked

testcross

• one individual of unknown genotype is crossed with another individual with a homozygous recessive genotype

• reveals the genotype of the first unknown individual

branch diagram method

used for solving genetic cross probabilities

How do make a branch diagram method?

1. break cross down into 2 monohybrid crosses

2. find the ratios of each of the resulting traits

3. combine the monohybrid ratios with the multiplication rule

pedigrees

pictorial representations of a family history, outlining the inheritance of one or more characteristics in humans

three ways to investigate characteristics in humans

1. twin studies - examine concordance of a trait between members of a twin pair

2. adoption studies - compare adopted persons with their adopted and biological parents

3. pedigrees

proband

individual who initiates the pedigree (indicated by a diagonal arrow)

what do roman numeral represent in pedigrees

generations

what do lines represent in pedigrees?

relationships

what do Arabic numbers represent in pedigrees?

identify individuals within a generation

What modes of inheritance should one recognize when looking at a pedigree?

1. dominant or recessive

2. sex-linked or autosomal or mitochondrial

in a pedigree, how can you tell that a trait is dominant

• generally, appears in every generation

• affected individuals usually have at least one affect parent

in a pedigree, how can you tell if a trait is recessive

• can skip generations

• affected person can be born from unaffected parents

in a pedigree, how can you tell if a trait is autosomal?

appears almost equally between both sexes

in a pedigree, how can you tell if a trait is sex-linked

it appears more in one sex than another

Y-linked trait

a trait that a father passes to all sons

What are sons considered because they inherit X traits from mom

hemizygous

in a pedigree, how can you tell if a trait is mitochrondrial?

trait has maternal lineage, NOT paternal