1. Genetics
Chapter 12

2. Gregor Mendel
Gregor Mendel was a monastery priest who carried out the first important studies of heredity
Heredity – the passing on of characteristics from parents to offspring
Genetics is the branch of biology that studies the patterns of inheritance and variations in organisms
Mendel was the first to succeed in predicting how traits would transfer from one generation to the next.

3. Mendel’s Experiment Mendel chose to study pea plants
They reproduce sexually – meaning they have two distinct sex cells (called gametes)
The female gamete is the ovule (located on the pistil)
The male gamete is in the pollen grain
The structure of the pea plants allowed Mendel to control their fertilization
The transfer of male pollen grains to the pistil is called pollination
Fertilization is when the male and female gametes unite
Mendel studied one trait at a time and analyzed the results mathematically

4. Monohybrid Crosses
Mendel crossed short pea plants with tall pea plants
Mendel called this cross a hybrid – a cross between two parents that show different forms of a trait
Mendel’s first experiments were monohybrid since the parents differed only in one trait.

5. Monohybrid Crosses
The first generation (P1) are the initial cross organisms
In this case, a short pea plant and a tall pea plant
The second generation (F1) includes the offspring from the initial cross
For Mendel, this resulted in all tall plants
The third generation (F2) is the result of crossing the offspring from the F1 generation
For Mendel, this resulted in 75% tall plants and 25% short

6. Mendel’s Conclusions
Mendel concluded that each organism had two factors that controlled each trait.
One factor is passed down from each parent
The factors are located on the chromosomes
The different forms of the genes are called alleles

7. Rule of Dominance
Even though the F1 generation plants had a tall allele from one parent and a short allele from the other, they all appeared tall
Mendel concluded that one trait was dominant over the other trait which he called recessive
Plants with two alleles for tallness (TT) were tall.
Plants with two alleles for shortness were short (tt)
Plants with one allele for tallness and one for short (Tt) were tall

8. Law of Segregation
If an organism has two different alleles for a trait, that organism can make two different types of gametes.
Tt plant can produce T gametes and t gametes
Fertilization from a Tt + Tt cross will result in random pairs of the available gametes (four possible combinations)
T from male + T from female = TT
T from male + t from female = Tt
t from male + T from female = Tt
t from male + t from female = tt

9. Describing Offspring
There are two ways to describe the results of a pairing
An offspring’s genotype
An offspring’s phenotype
Genotype – the gene combination of an organism
A pea plant with two alleles for tallness has the genotype, TT
A pea plant with one allele for tallness and one allele for shortness has the genotype, Tt
Phenotype – the physical appearance of the organism
An pea plant with two alleles for tallness has the phenotype, tall
An pea plant with one allele for tallness and one allele for shortness has the phenotype, tall

10. Describing Offspring
T t
T T
If an organism has the same two alleles for a trait, the organism is homozygous TT tt
If an organism has two different alleles for a trait, the organism is heterozygous Tt
Recessive traits must be homozygous (tt) to be expressed
Dominant traits will display if they are homozygous (TT) or heterozygous (Tt)
t t

11. Predicting Offspring
Mendel devised a way to predict the possible outcome of a cross. This method is called a Punnett Square (or Punnett Chart)
Punnett Square’s take into account that fertilization of gametes is random
To use a Punnett Square, you need to know the genotypes of the parent generation.
Punnett Squares can be used to predict offspring from a monohybrid or a dihybrid cross

12. Monohybrid Punnett Square
Put one of each type of possible gamete from one parent on top of the square
Put one of each type of possible gamete from the other parent on the side of the square
Fill each box with the gamete of that box’s row and column
The possible offspring combinations can be seen
The ratio of offspring phenotypes after a heterozygous monohybrid cross is 3:1
In this case we see 3 Tall and 1 Short TT Tt tt

13. Monohybrid Crosses t t T T T Tt Tt T TT TT t tt tt t Tt Tt
Homozygous tall crosses with heterozygous tall
Homozygous short crosses with heterozygous tall t t T T T Tt Tt T TT TT t tt tt t Tt Tt
Genotype Ratio: 2TT : 2Tt
Phenotype Ratio: All Tall
Genotype Ratio: 2Tt : 2tt
Phenotype Ratio: 2 Tall : 2 Short

14. Incomplete Dominance
Incomplete dominance is a condition in which one allele is not completely dominant over another.
The phenotype expressed is somewhere between the two possible parent phenotypes.
In snapdragons, flower color is controlled by incomplete dominance. The two alleles are red (R) and white (r). The heterozygous genotype (Rr) is expressed as pink.

15. Dihybrid Crosses
Mendel also performed dihybrid crosses – involving two traits
He wondered if the two traits would be inherited together.
His P1 generation crossed round yellow peas (RRYY) with wrinkled green peas (rryy)
The F1 generation was all round and yellow
The F2 generation had 9 round yellow, 3 round green, wrinkled yellow and 1 wrinkled green

16. Law of Independent Assortment
The fact that traits for the color of the pea and the shape of the pea were passed on independently of each other led to the Law of Independent Assortment
When a pea plant with the genotype RrYy produces gamete, the alleles R and r will separate from each other (Law of Segregation) as well as the from the Y and y alleles (Law of Independent Assortment)
Alleles will sort independently unless they are “linked”. This usually occurs when they are so close to each other on the chromosome that they one is rarely passed on without the other.

17. Dihybrid Punnett Square
Put one of each type of possible gamete combination from one parent on top of the square
Put one of each type of possible gamete combination from the other parent on the side of the square
Fill each box with the gamete of that box’s row and column
The possible offspring combinations can be seen
The ratio of offspring phenotypes after a heterozygous dihybrid cross is 9:3:3:1
In this case we see 9 round yellow, round green, 3 wrinkled yellow and wrinkled green

18. Dihybrid Punnett SquareRY RY RY RY
Homozygous round and yellow with a heterozygous yellow and round
RRYY with RrYy
Offspring are all round and yellow RY
RRYY
RRYY
RRYY
RRYY Ry
RRYy
RRYy
RRYy
RRYy rY
RrYY
RrYY
RrYY
RrYY ry
RrYy
RrYy
RrYy
RrYy

19. Dihybrid Punnett Square
Homozygous wrinkled and green with a heterozygous yellow and round
rryy with RrYy
4 round yellow, round green, wrinkled yellow, 4 wrinkled, green ry ry ry ry RY
RrYy
RrYy
RrYy
RrYy Ry
Rryy
Rryy
Rryy
Rryy rY
rrYy
rrYy
rrYy
rrYy ry
rryy
rryy
rryy
rryy

20. Sex Determination
Remember that humans have 22 pairs of autosomes and 1 pair of sex chromosomes
These sex chromosomes determine the gender of the offspring
XX is a female
XY is a male
Each offspring gets an X from the mother and either an X or a Y from the father

21. Sex Determination
Predicting the sex of the offspring can be done using a Punnett Square
Each time a male gamete fertilizes a female gamete, there is a 50% chance for either sex

22. Sex-Linked Traits
Traits controlled by genes carried on the X or Y chromosomes are called sex-linked traits
Most of these types of traits are carried on the X chromosome
The alleles for different forms of the sex-linked traits are shown as superscripts on the X
XRXR XRXr XrXr
XRY XrY
Because the Y does not carry an allele, a male could not be heterozygous for a sex-linked trait

23. Predicting Sex-Linked Traits
The chances that an offspring will have a sex-linked trait can be predicted using a Punnett Square

24. Sex-Linked Crosses XN Y Xn Y XNXn XnY XNXn XNY XNXn XnY XnXn XnY
Colorblindness is a recessive sex-linked trait. Use XN for the normal allele and Xn for the colorblind allele
Heterozygous Normal mother and Colorblind father
Colorblind mother and Normal father XN Y Xn Y Xn
XNXn
XnY XN
XNXn
XNY Xn
XNXn
XnY Xn
XnXn
XnY
Normal daughters (carriers)
Colorblind sons
50% Normal daughters and sons
50% colorblind daughters and sons

25. Blood Type
Human blood types demonstrate multiple alleles (more than two alleles of the gene)
A, B, O
Human blood types also demonstrate codominance – where heterozygous alleles can be expressed equally
A and B are codominant
O is recessive
These alleles are written as IA, IB, and i
IAIA or IAi will have type A blood
IBIB or IBi will have type B blood
IAIB will have type AB blood
ii will have type O blood

26. Blood Type Crosses IA IB IB IB IAi IBi IAIB IAIB IAi IBi IAIB IAIB
Homozygous Type A mother and Homozygous Type B father
Type O mother and Type AB father IA IB IB IB i
IAi
IBi IA
IAIB
IAIB i
IAi
IBi IA
IAIB
IAIB
50% blood type A
50% blood type B
100% type AB

27. Polygenic Traits
Polygenic traits are traits that are controlled by two or more genes. These traits often show a great variety of phenotypes, e.g. skin color.

28. Pedigrees
A pedigree is a chart constructed to show an inheritance pattern (trait, disease, disorder) within a family through multiple generations.
Through the use of a pedigree chart and key, the genotype and phenotype of the family members and the genetic characteristics (dominant/recessive, sex-linked) of the trait can be tracked.
An example of a pedigree key:

29. Pedigrees
Family with a dominant autosomal genetic trait

30. Pedigrees
Family with a recessive sex-linked genetic trait