Ch 14 Mendelian Genetics

blending theory of genetics

antiquated belief that inheritance is a mix of the mothers and fathers traits → offspring end up with an intermediate characteristic

• before Mendel

Gregor Mendel

father of genetics

• discovered the fundamental rules that govern inheritance in 1860s

• tested genetics experimentally and quantitatively - trained in botany and statistics

• worked with garden peas (many varieties, easy to control reproduction, lots of offspring) using artificial crossing

• findings and theories not widely accepted until the 1900s, eventually proven by the discovery and studying of meiosis and gamete formation, Mendel theorized this long before

artifical cross

manually transferring the male, sperm parts to the female egg parts to facilitate reproduction

• Mendel’s peas

character

Inheritable feature of an organism

• ex: eye color, flower color

trait

variant of an inheritable character

• ex: blue eyes, purple flower

true breeding

strain that always produces offspring identical to parents

monohybrid cross

examining the patterns of inheritance in different variants of the same trait

• Mendel’s crossing of the purple and white-flowered plants (P generation) produced offspring of all purple flowers (F1 generation)

PP x pp

homozygous dominant crossed with a homozygous recessive

yields 100% dominant offspring

Pp x Pp

heterozygous cross

yields 3:1 ratio of dominant phenotype to recessive

Pp x pp

heterozygous crossed with homozygous recessive

yields 1:1, dominant phenotype to recessive

dominant trait

trait, coded for by genotype, always shows in the phenotype

• ex: brown eyes, purple flower color

recessive trait

trait coded for in genotype that only shows in phenotype if two alleles are present, can be masked by the dominant allele if it’s present

• ex: blue eyes, white flower color

alleles

alternative forms of the same gene, one from each parent on each gene that codes for a trait on each side of a homologous chromosome, directly connected to locus on a chromosome where certain nucleotides determine gene expression

• recessive or dominant

Law of segregation

Mendel’s conclusion that 2 copies of each hereditary factor segregate during gamete formation so that offspring acquire one factor from each parent

→ paired condition restored at fertilization, half the gametes carry one allele, half carry the other

punnet square

uses statistics to show all possible combinations of allele/phenotypes in offspring based on parent’s genotype

• each square is an equally as probable product of fertilization

homozygous

2 of the same allele for a trait, true breeding

• pp or PP

heterozygous

having 2 different alleles for a trait

• Pp

phenotype

expressed trait

• purple or white

genotype

genetic makeup of an organism

dihybrid cross

mate parents with 2 different characters, 4×4 punnent square

• heterozygous parent → 4 gametes, YR, Yr, yR, yr

• double heterozygous parent x double heterozygous parent → 9:3:3:1 ratio of phenotypes

law of independent assortment

each allele pair segregates independently during gamete formation

determined by arrangement of chromosomes during metaphase, random

chromosomal theory of inheritance

genes are located on chromosomes and it is chromosomes that segregate and assort independently during gamete formation

• chromosomes occur in pairs, as do alleles of each gene

• chromosomes of each pair are separated and delivered singly to gametes, as are alleles of a gene

• fertilization: one member of each chromosome (one from mom, one from dad) → 2 alleles of each gene

• discovered as a result of X-linked genes by T.H. Morgan

sex-linked genes

genes carried on sex chromosomes, specifically X-chromosomes

→ males only have one allele to contribute from x gene, higher likelihood of receiving and expressing x-linked recessive gene

→ gender based differences in inheritance bc males are XY and females are XX

• discovered because certain traits show up more in males than females, vv

linked

genes are located on the same chromosomes and tend to be inherited together

• do not assort independently - move together in meiosis and fertilization

• linkage disequilibrium - determines phenotype linkage as well

• only separate when crossing over or genetic recombination occurs

linkage disequilibrium

when genes are located on the same chromosome and determines whether phenotypes are linked in inheritance patterns

genetic recombination

linkages broken between genes on same chromosome → produces offspring with more combinations of traits

→ along with crossing over, only way linked genes separate

locus

location of gene on a chromosomes

• aka the particular DNA sequence that typically encodes a protein responsible for a phenotype → an allele’s expression here is determined by the DNA sequence

• different allese consist of differences in DNA sequence of a gene which may result in functional differences in the protein encoded by the gene

incomplete dominance

heterozygote has an intermediate phenotype

• genotypic and phenotypic ratios are the same, not blending

• arex: flower color in carnations

codominance

both alleles are fully expressed in the heterozygote

• ex: MN blood type

multiple alleles