Genetics

Some Vocab.

Genotype – allele combination

Capital letter = dominant allele

Lowercase letter = recessive allele

Ex – AA, Aa, aa

Phenotype – physical appearance

Ex – green, yellow

Some Vocab.

Homozygous – two alleles same

Homozygous dominant: AA

Homozygous recessive: aa

Heterozygous – two alleles different

Aa

Gregor Mendel

“Father” of genetics

Austrian monk, mid-1800s

Researched pea plant inheritance

Easy to grow, fast reproduction

Studied plant height, pea shape/color,

pod color, etc.

Pea Plant Reproduction

Self-pollination

Male gametes (pollen) fertilize egg of

same flower

Produces pure-bred offspring

True-breeding: produce offspring

identical to parent when self-

pollinated

Pea Plant Reproduction

Cross-pollination

Pollen from one plant fertilizes egg of

another plant

Offspring have two parents

Mendel’s Experiments

P = Parent generation

F1 = First filial generation

F2 = Second filial generation (F1 X F1)

Monohybrid cross: Cross involving one trait

P Pure Purple X Pure White

F1 All Purple

F2 3 Purple:1 White

Dihybrid Cross

Mendel looked at the inheritance

patterns of two traits

Seed shape and seed color

Found that the traits were inherited

independently of each other

P Pure Yellow/Smooth X Pure Green/WrinkledF1 All Yellow/SmoothF2 9 Yellow/Smooth:3 Yellow/Wrinkled:3 Green/Smooth:1 Green/Wrinkled

http://vie

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Mendel’s Conclusions

1. Law of Dominance: one allele

(form of a gene) is dominant, one

is recessive

Recessive trait was hidden in F1

generation

Purple = dominant

White = recessive

Mendel’s Conclusions

2. Law of Segregation:

alleles for a gene

separate when gametes

form

Due to separation of

homologous chromosomes

during Anaphase I

Each gamete gets one

copy of each gene

Mendel’s Conclusions

3. Law of Independent Assortment

Genes on separate chromosomes are

inherited at random

Due to random chromosome shuffling

in Metaphase I

Independent Assortment

Metaphase I

Punnett Squares

Used to show probability of certain

genetic outcomes in offspring

First must determine possible gametes

Heterozygous tall plant = Tt

Half of gametes will get ‘T’, the other half will

get ‘t’

Homozygous tall plant = TT

All gametes will get ‘T’

Punnett Squares

Gametes go on the

top and side

Combine gametes to

find possible

offspring

Punnett Squares

Genotype ratio

1TT: 2Tt: 1tt

Phenotype ratio

3 tall: 1 short

Tt X Tt

Non-Mendelian Genetics

Not all traits follow Mendel’s Law of

Dominance

Four Variations

Incomplete Dominance (blending)

Codominance (two phenotypes)

Multiple Alleles

Polygenic Traits

Incomplete Dominance

Neither allele is dominant, only 50%

of protein is produced

Heterozygous phenotype is a blend

of both homozygous phenotypes

Ex – wavy hair, pink flowers

Incomplete Dominance

Codominance

Neither allele dominates the other, both

produce a protein

Heterozygous phenotype is a combination

of both homozygous phenotypes

Ex – checkered chicken, human blood

types, roan cattle

Multiple Alleles

Some genes have more than two

alleles

Each individual only inherits two,

but there are more than two in the

population

Ex – Rabbit fur color

Multiple Alleles

Rabbit fur alleles (in order of dominance)

C: dark gray

cch: chinchilla

ch: himalayan

c: albino

Multiple Alleles

Polygenic Traits

Many traits are

controlled by more than

one gene

Traits show wide

variation

Ex – human height, eye

color, bell pepper colors

http://www.ck12.org/biology/Polygenic-Traits/lesson/Polygenic-

Traits-Advanced-BIO-ADV/

Linked Genes

Each chromosome contains 100s of genes

Those genes are said to be linked, because

they are inherited together

If genes are far apart on chromosome, crossing

over will happen frequently between them

Genes are mapped based on rate of

crossing over

Linked Genes

Genes A & B are completely linked

Will not show Independent

Assortment

Genes A & C are incompletely linked

Crossing over frequent

Genes A & D are unlinked

Will show Independent Assortment