1. Heredity & Genetics
Chapter 8
CP Biology

2. Heredity
Heredity, or inheritance, is the passing down of something from one generation to the next
We inherit our traits from our parents
Traits include things like:
Hair color
Eye color
Height
Freckles
Dimples
Widow’s peak
Hitchhiker’s thumb
The list goes on…

3. Genetics Genetics is the science of heredity.
Recall that our chromosomes come in pairs called homologues:
Each homologous pair contains one maternal chromosome (inherited from mom) and one paternal chromosome (inherited from dad).
Homologous chromosomes contain the same genes, but may have different versions of some genes.
Different versions of the same gene are called alleles.

4. Genetics
For example, you may inherit the allele for freckles from your mother, but the allele for no freckles from your father.
So what determines which trait you actually end up with?
Sometimes one allele is completely dominant over another.
Freckles are a dominant trait, so even with just one allele for freckles, you will have freckles.
Other alleles share dominance, so the trait you see could be a blend of the two.
Many traits (such as eye color) are determined by more than one gene.

5. Origins of Genetics
Gregor Mendel is considered to be the father of modern genetics.
He was a monk who studied heredity in pea plants.
Pea plants are ideal for studying heredity because:
They reproduce quickly
They have many offspring
They have many easily observable characteristics:
Plant height, flower color, seed color and shape, pod color, etc.

6. Gregor Mendel
Mendel studied 7 characteristics in pea plants:

7. Mendel’s Experiments Pea plants normally self-pollinate
The pollen from one flower fertilizes the seeds of the same flower
After many generations, the offspring still have the same traits as the parent plants
These are called pure plants – they are homozygous (can be dominant or recessive)
Mendel cross-pollinated pure pea plants with opposite characteristics
Used pollen from purple flowers to pollinate plants with white flowers, and vice versa
In both cases, all of the offspring had purple flowers
When these purple offspring plants were allowed to self-pollinate, they produced both purple-flowered and white-flowered plants

8. Parental & Filial Generations
The first plants that are crossed are the parental (P) generation – these plants are pure (true- breeding) plants
The offspring of a parental cross are the first filial (F1) generation – these plants are called hybrids and all exhibit the dominant phenotype
The offspring of the F1 plants are the F2 generation – there are 3 dominant phenotypes for every 1 recessive phenotype

9. Punnett Squares
Punnett squares are the tool we use to predict the genotypes and phenotypes in the offspring of a given set of parents.
The genotype of one parent goes across the top
The genotype of the other parent goes down the side
The parent genotypes are crossed to determine the possible genotypes of the offspring F f Ff ff f f Ff ff

10. Punnett Squares and Probability
When 2 heterozygous organisms are crossed, we expect:
A genotype ratio of 1:2:1
A phenotype ratio of 3:1
When working with Punnett squares, we are dealing with probability.
The actual results may not exactly match what the Punnett square predicts
The larger the number of offspring, the more closely the actual results match what is predicted.

11. Law of Segregation
Mendel’s Law of Segregation states that during gamete formation (meiosis), allele pairs separate (segregate), and randomly unite during fertilization.
To test this hypothesis, he crossed a heterozygous plant with a homozygous recessive plant.
He predicted that there would be a 50:50 ratio of purple to white plants.

12. This principle can also be used to figure out if an organism is homozygous or heterozygous for a dominant trait

13. Independent Assortment
The principle of independent assortment:
Traits are inherited independently of one another
This can be shown by a two-trait cross
When both parents are heterozygous for both traits, it is called a dihybrid cross.
In this example, both parents are RrYy:
R = round seeds
r = wrinkled seeds
Y = yellow seeds
y = green seeds

14. Dihybrid Crosses 9 round, yellow seeds 3 round, green seeds
(both dominant traits)
3 round, green seeds
(one dominant trait, one recessive trait)
3 wrinkled, yellow seeds
1 wrinkled, green seed
(both recessive traits)
The phenotype ratio of a dihybrid cross is always 9:3:3:1

15. Incomplete Dominance
In incomplete dominance, the heterozygous phenotype is intermediate between the two alleles
Ex: Some flowering plants have an allele for red flowers and an allele for white flowers. Plants that have one of each allele will have pink flowers.
Since neither allele is recessive, both are written as capital letters.

16. Codominance
In codominance, neither allele is dominant, and organisms that are heterozygous will express both phenotypes equally.
Ex: A species of chicken has an allele for black feathers and an allele for white feathers. Chickens that have one of each allele will have both black and white feathers (NOT gray feathers, as you would expect in incomplete dominance)
As in incomplete dominance, both alleles are written as capital letters.

17. Incomplete Dominance vs. Codominance

19. Phenotype (Blood Type)
Multiple Alleles
There may be more than 2 possible alleles for a trait
Ex: Human blood type
3 possible alleles: IA, IB, and i
i is recessive
IA and IB are codominant
Genotype
Phenotype (Blood Type)
IAIA or IAi A
IBIB or IBi B
IAIB AB ii O

20. ABO Blood Types

21. The Chromosome Theory of Heredity
In the early 1900s, Walter Sutton proposed the chromosome theory of heredity:
Genes are carried on chromosomes
This theory was confirmed by Thomas Hunt Morgan, who discovered the X and Y chromosomes while studying fruit flies!
X and Y chromosomes are called sex chromosomes
Females: XX
Males: XY
All other chromosomes are called autosomes (22 pairs in humans)

22. Sex-Linked Inheritance
The X and Y chromosomes contain other genes besides the ones that determine sex.
The X chromosome is larger and contains more genes than the Y chromosome.
A male needs only one recessive sex-linked allele to have the recessive phenotype

23. Sex-Linked Inheritance
Sex-linked alleles are written as superscripts on the X chromosome; the Y chromosome gets no superscript.
Ex: Eye color in fruit flies
Red eye allele (dominant): R White eye allele (recessive): r
Red-eyed females can be XRXR or XRXr
White-eyed females must be XrXr
Red-eyed males must be XRY
White-eyed males must be XrY

24. Color Blindness is a Sex-Linked Trait

25. Many Traits Are Determined By More Than One Gene
A polygenic trait is a single trait that is determined by multiple genes
Results in continuous variation – the possibility of many phenotypes from one extreme to another
Modifier genes affect the expression of other genes
Ex: Human eye color – a base color of either brown (B) or blue (b) is determined by one gene, but modifier genes can give people eyes that are green, hazel, gray, etc.