MORE ON GENETICS

MORE AFTER MENDEL

• Mendel only studied traits that had one dominant allele and one recessive allele.• There is more to inheriting traits than just

dominant and recessive alleles.

INCOMPLETE DOMINANCE

• Incomplete dominance occurs when two alleles do not have complete dominance so the resulting trait is a mix of the two alternate traits• Ex: crossing red snapdragon flowers with white

snapdragon flowers produces pink snapdragon flowers.

PUNNETT SQUARE FOR INCOMPLETE DOMINANCE

• When using Punnett squares to illustrate incomplete dominance, you do not use lowercase letters since neither allele is recessive to the other one. • A capital letter is used for one allele, and the

same capital letter with an apostrophe or tic mark after it represents the other allele. • ex: RR’

EXAMPLE INCOMPLETE DOMINANCE

R R

R’ RR’ RR’

R’ RR’ RR’

R- Red alleleR’- White alleleRR’- Pink flowers

INCOMPLETE DOMINANCE

• How can you tell if you are looking at a cross that shows incomplete dominance?• There are three different phenotypes• One for each parent • One for the offspring

• The phenotype of the offspring will be a mix or blend of the parents’ phenotypes.

CODOMINANCE

• Codominance expresses both alleles equally. • Both alleles are dominant• Both parental phenotypes appear in the offspring

together but not mixed. • Ex: In certain kinds of chickens, black feathers and

white feathers have codominant alleles. When you cross them you get offspring that are black and white speckled.

• Ex: Roan color in cattle and horses. RR produces all red hairs and WW produces all white hairs. RW produces roan fur, which has red and white hairs all mixed together.

PUNNETT SQUARE FOR CODOMINANCE

• Codominance doesn’t use lowercase letters in Punnett squares.• Sometimes superscripts are used to illustrate

codominance• Ex: FW or FB

PUNNETT SQUARE EXAMPLE CODOMINANCE

FB FB

FW FBFW FBFW

FW FBFW FBFW

FB – black feather alleleFW – white feather alleleFBFW – speckled feathers

SICKLE CELL ANEMIA

• Is a genetic disease that is caused by a codominant gene. • The trait causes the hemoglobin in red blood cells

to be the wrong shape. Hemoglobin is the protein that carries oxygen. • If it doesn’t have the right shape, the red blood

cell changes shape to a crescent shape.• The deformed cells can stick together and block

small blood vessels.

SICKLE CELL ANEMIA

• Most common in people of African ancestry. Although, it can be found in Mediterranean and Middle Eastern ethnic groups.

• About 2 million Americans carry one gene for the sickle cell trait.

• A person that is heterozygous for the sickle cell gene, then they produce enough normal red blood cells to live normal lives.

• People who are homozygous for sickle cell have very serious health problems. It causes the person to be tired and weak, and increases the risk of stroke and infections. They also become anemic since the sickle cells die quickly

MULTIPLE ALLELES

• Although each individual will have only two alleles per trait, it is possible to have more than two alleles that can determine a trait. • Ex: human blood type• Humans have 4 main blood types: A, B, AB, and O.• These blood types are determined by multiple alleles. • There are 3 alleles for the blood type gene instead of

2, but you have only two in your cells. • One from your mother and one from your father

• How do you get four blood types from three alleles?

HOW MULTIPLE ALLELES WORK

• 1. Each person has two alleles – one from the mother and one from the father.• 2. There are three alleles for blood type – IA, IB,

and i.• 3. IA and IB are codominant to each other.• 4. Both IA and IB are dominant over i, which is

recessive.

BLOOD TYPES

Blood Type Genotypes

A IAIA or IAi

B IBIB or IBi

AB IAIB

O ii

BLOOD TYPING

• Blood typing is important because a person getting a blood transfusion can die if given the wrong type of blood.• It helps determine paternity of a father to a child• Ex: If a child has AB blood and the mother has

type A blood, couls a man with type O blood be the father?• Child’s genotype is IAIB and the man’s genotype is ii. The

mother’s genotype is either IAIA or IAi. This man could not be the father because he could not give the child an IB allele to make the AB blood type.

LINKED GENES

• Remember the Law of Independent Assortment explains how traits are inherited independently of other traits• There are some exceptions to this law.• The law is true with the genes are on different

chromosomes, but not always true if they are on the same chromosome. • Genes that are close enough together on the

chromosome may be linked, meaning they are inherited together.

THOMAS MORGAN

• Thomas Morgan was a scientist in the early 1900’s that studied an insect called Drosophila, or fruit fly.

• Fruit flies do not show independent assortment.• They could either have a grey body (G) or a black

body (g) and could have long wings (L) or short wings (l) called vestigial wings.

• He found that the grey flies tended to have long wings and the black flies tended to have vestigial wings.

• Those genes were close on the same chromosome and tended to be inherited together.

• Humans also have linked genes such as the genes for red hair and freckles.

SEX CHROMOSOMES

• Humans have 23 pairs of chromosomes (total of 46 chromosomes). • Humans have 22 pairs of autosomes and one pair

of sex chromosomes. • The sex chromosomes are called X and Y and

they determine gender. • Females have two X chromosomes (XX) and

males have one X and one Y (XY).• The X chromosome is larger and contains more

genes.

SEX-LINKED GENES

• The X and Y chromosomes determine gender, but they also have genes on them that determine other characteristics. • Those genes are called sex-linked genes, since

they are found on either the X or the Y chromosomes. • X is larger so it contains more sex-linked genes

called X-linked genes. • Don’t confused sex-linked with linked genes.

Linked genes are on autosomes.

COLOR BLINDNESS

• Defects on either the X or the Y chromosome may lead to a sex-linked disorder or disease.

• Red-green color blindness is a common sex-linked disorder.

• Color blindness (color deficiency) occurs when there is a problem with the cone cells in the eye, which sense colors.

• Red-green color deficiency creates difficulty determining the difference between red and green.

• Blue-yellow deficiency can be mild that it goes unnoticed or so severe there is no perception of color.

COLOR BLINDNESS

• Both males and females can be color-deficient although it is rare in females.• The gene is carried on the X chromosome and is

recessive. • It would have to be inherited from both the

mother and the father for a female to be color deficient• Males only have one X chromosome so if it is

inherited from the mother he will have color deficient vision. • 10% of males have some form of color deficiency.

HEMOPHILIA

• Human blood contains proteins called clotting factors that stop bleeding with a person gets a cut or a scratch.• People with hemophilia don’t have enough of

these proteins and will bleed for longer periods of time than those with normal levels of the protein. • This gene is carried on the X chromosome and is

more common in males. • Women are carriers for the trait.