NMAT Biology | Genetics

Gene

A segment of DNa that codes for a trait

Allele

Different versions of a gene

Genotype

Genetic make up

ex: AA, Aa, aa

Phenotype

Physical appearance

Homozygous

2 identical alleles (AA or aa)

Heterozygous

Two different alleles (Aa)

Dominant

Expressed if at least one copy is present

Recessive

Expressed only if two copies are present

Law of segregation

Mendel’s first law

Two alleles for a gene separate during meiosis, so each gamete receives only 1 allele

ex: Parent is Aa, and the gametes are A or a, but never Aa

Law of independent assortment

Different genes separate independently

2 traits can be assessed at once

Dihybrid crosses

Both parents have hybrids for both genes

Phenotypic ratio = 9:3:2:1

ex: BbTt (Phenotype would be a brown-eyed', big-toothed person)

2 rules of probability

1. AND

• ex: If the probability being a boy is ½ and having blue eyes is 1/4 ; you multiply both and get 1/8

2. OR

• ex: You have AA and Aa, ¼ and ½ respectively; if you’re going to use OR, you have to add them

Non-mendelian genetics

1. Incomplete dominance

2. Codominance

3. Multiple alleles

4. Polygenic traits

5. Pleiotropy

Incomplete dominance

Parents blend traits

ex: Roses and snapdragons

Codominance

Both traits show

ex: Blood type AB where both A and B are expressed; not blended

Multiple Alleles

More than 2 alleles exist

ex: ABO blood groups

Polygenic traits

Many genes control one trait

ex: Heigh, skin color, eye color

Pleiotropy

One gene affects many traits

ex: Sickle cell disease

Blood types

Sex-linked Inheritance

X-linked recessive

ex: Color blindness, hemophilia— the traits appear more in males because they only have one X chromosome

Note: No male-to-male transmission since fathers give sons a Y chromosomes and not X

Genetic mapping

Figuring out what genes are close together