1. Patterns of Inheritance
3.3
Tigers have different color
variants due to genetic conditions. Here is a ’normal’ tiger pictured
with three color variants: a golden tiger whose stripes are a darker
orange, not black; a white tiger
with black stripes; and a
stripeless white tiger.

2. Codominance and Incomplete Dominance
If a gene has 2 alleles, one may not dominate over the other.
Codominance-alleles are equally dominant
Incomplete dominance- not completely dominant
Codominance (left) and incomplete dominance
(right)

3. R W RR RW WW R W Incomplete dominance
When one allele is NOT completely dominant over another (they blend) – incomplete dominance
Example: In carnations the color red (R) is incompletely dominant over white (W). The hybrid color is
pink. Give the genotypic and phenotypic ratio from a cross between 2 pink flowers.
RW X RW R W RR RW WW R W
Genotypic = RR : 2 RW : 1 WW
Phenotypic = 1 red : 2 pink : 1 white

4. When both alleles are expressed – Codominance
Example: In certain chickens black feathers are codominant with white feathers.
Heterozygous chickens have black and white speckled feathers.

5. Multiple Alleles Many genes have more than 2 alleles. Example Blood
ABO
A and B are codominant
A and B are both dominant over O
ABO genotypes and phenotypes
AA Type A
AO Type A
BB Type B
BO Type B
AB Type AB
OO Type O

6. Children would be type A or B only
Example: What would be the possible blood types of children born to a female with type AB blood and
a male with type O blood?
AB X OO A B AO BO O O
Children would be type A or B only

7. Polygenetic Traits
Some traits are controlled by more than one gene. This is called a Polygenetic trait.
They may or may not be on the same chromosome.
Skin color
Height

8. Environmental Influences
Genes play an important role in an organisms traits, but phenotypes are influenced by the environment as well.
Skin color= genes, but sun exposure can change the color.

9. Human Sex Chromosomes
Animals and most plants have two special chromosomes. They are called sex chromosomes.
These are chromosomes that determine the sex of the organism. All of the other chromosomes are called autosomes.
Genes on sex chromosomes may be inherited differently than genes on autosomes.

10. Human Sex Chromosomes
In people, the sex chromosomes are called X and Y chromosomes.
Individuals with two X chromosomes are normally females. XX
Individuals with one X and one Y chromosome are normally males. XY
Moms always pass an X chromosome.
Dads give an X to daughters and a Y to sons.

11. Sex-linked Traits
Traits controlled by the sex chromosomes are called sex-linked traits.
One gene on the Y chromosome determines the male gender. The Y chromosome is the smallest human chromosome and carries very few other traits.
The X chromosome is much bigger and does have a lot to traits called X-linked.

12. X-linked
X-linked traits are different than other traits because males only have 1 allele for the trait carried on the X chromosome they got from their mom. Any recessive x-linked allele is expressed in males.
Females get a second allele for the trait on the X chromosome they get from their dad, just like any other autosomal trait.

13. Sex-linked color blindness
In this example, both parents are able to see red and green colors. However, because the mother has one allele for color blindness she is called a carrier for the trait. She passes the allele to half of her children. One daughter is a carrier, and one son has the color blindness trait. No matter how many children this couple has, none of the daughters will have color blindness, but half of the sons, on average, will have the trait. Can you explain why?

14. Color Blindness In Humans: An X-Linked Trait
Numbers That You Should See If You Are In One Of The Following Four Categories: [Some Letter Choices Show No Visible Numbers]
Sex-Linked Traits: 
Normal Color Vision:  A: 29,  B: 45,  C: --,  D: 26  
Red-Green Color-Blind:  A: 70,  B: --,  C: 5,  D: --  
Red Color-blind:  A: 70,  B: --,  C: 5,  D: 6  
Green Color-Blind:  A: 70,  B: --,  C: 5,  D: 2

15. hemophilia – blood won’t clot

16. Sex-Linked Disorder
Duchenne muscular dystrophy, affects about one out of every 3,500 males born in the United States. People with Duchenne muscular dystrophy rarely live past their early 20s. The disease is characterized by a progressive weakening of the muscles and loss of coordination. Researchers have traced the disorder to the absence of a key muscle protein called dystrophin and have tracked the gene for this protein to a specific locus on the X chromosome.
Posture changes during progression of Duchenne muscular dystrophy.

17. Mutations Mutations are changes to an organism’s DNA.
They occur naturally as the cell progresses through the cell cycle or as a result of environmental factors.
Mutations are an important part of the study
of heredity, as they are the ultimate source of
variation in a population.

18. Mutations can be harmful, beneficial or silent.
Silent mutations have no effect on an organism’s phenotype.
Whether or not a mutation is considered harmful or beneficial is determined by whether it helps an organism survive and
reproduce.
Mutations that are harmful will often be eliminated over time, as organisms do not survive or successfully reproduce.
Beneficial mutations, on the other hand, will often spread through a population.
Mutations are an important driver of diversity in organisms.
Mutations are the source of variation in our world and play
a key role in the evolution of species.

19. Examples:Down’s syndrome – (Trisomy 21) 47 chromosomes, extra chromosome at pair #21

20. Turner’s syndrome – only 45 chromosomes, missing a sex chromosome (X)
Girls affected – short, slow growth, heart problems

21. Klinefelter’s syndrome – 47 chromosomes, extra X chromosomes (XXY)
Boys affected – low testosterone levels, underdeveloped muscles, sparse facial hair

22. Examples:
Recessive gene mutations:
Sickle cell anemia – red blood cells are sickle shaped instead of round and cannot carry enough oxygen to the body tissues – heterozygous condition protects people from malaria

23. Phenylketonuria (PKU) – an amino acid common in milk cannot be broken down and as it builds up it causes mental retardation – newborns are tested for this
Dominant gene mutations:
Huntington’s disease – gradual deterioration of brain tissue, shows up in middle age and is fatal
Dwarfism – variety of skeletal abnormalities

24. Lesson Summary
• Inheritance is more complex for traits in which there is codominance or incomplete dominance. Traits may also be controlled by multiple alleles or multiple genes. Many traits are influenced by the environment as well.
• Sex chromosomes determine sex in animals and many plants. Other chromosomes are called autosomes.
Sexlinked traits are controlled by genes on sex chromosomes. They may be inherited differently than autosomal traits.
• Mutations can alter a gene and are the original source of new variations.

25. Lesson Review Questions
Recall
1. What are codominance and incomplete dominance? Give an example of each.
2. What is the difference between a multiple allele trait and a polygenic trait?
Apply Concepts
4. In Japanese four o’clock plants red (R) color is incompletely dominant over white (r) flowers, and the heterozygous
condition (Rr) results in plants with pink flowers. Use a Punnett square to determine the possible
offspring genotypes for a cross between a red plant and a white plant.
Think Critically
5. Explain why it is the father rather than the mother who determines the sex of their offspring.
Points to Consider
Genetics began with the rediscovery of Mendel’s laws in There have been many advances in genetics since
then.
1. What are some recent advances in genetics?
2. What do we now know about human genes?