1. Mendel and Punnett Squares
Genetics
Mendel and Punnett Squares

2. x = What is Genetics? or or
The study of heredity, how traits are passed from parent to offspring. or x = or

3. Gregor Mendel
The study of genetics started with Gregor Mendel and his pea plant garden.
He was an Austrian Monk that lived in the mid 1800s.
He observed pea plants and how they passed their genetic information on to produce different pea plants.

4. Pea Plants
Mendel’s cross between tall pea plants yielded all tall pea plants. His cross between small pea plants yielded all small pea plants.
Mendels’ cross between tall pea plants and small pea plants yielded all tall pea plants = X = X = x

5. Mendel then crossed these second generation tall pea plants and ended up with 1 out 4 being small.x =

6. GENES
Mendel’s work led him to the understanding that traits such as plant height are carried in pairs of information not by single sets of information.
Carrying the information are chromosomes.
Chromosomes are made up of DNA and divided into sections called genes.

7. Genetics
Small sections of DNA are responsible for a “trait”. These small sections are called “Genes”.
Gene - A segment of DNA that codes for a specific trait
Trait - A characteristic an organism can pass on to it’s offspring through DNA

8. Genetics There are three basic kinds of genes:
Dominant - A gene that is always expressed and hides others
Recessive - A gene that is only expressed when a dominant gene isn’t present
Codominant - Genes that work together to produce a third trait
Descriptions:
P generation
F 1 generation
F2 generation

9. Human Traits Dominant Recessive Earlobe attachment Unattached Attached
Tongue Rolling
Can roll the tongue
Cannot roll
Dimples
Have dimples
Do not have dimples
Handedness
Right handed
Left handed
Hand clasping
Left thumb
Right thumb
Hairline
Widow’s peak
Straight hairline

10. Genetics A dominant gene will always mask a recessive gene.
Dominant and Recessive Genes
A dominant gene will always mask a recessive gene.
A “widows peak” is dominant, not having a widows peak is recessive.
If one parent contributes a
gene for a widows peak, and the other parent doesn’t, the
off- spring will have a widows peak.
Widows Peak

11. Genetics
Punnett Square - A tool we use for predicting the traits of an offspring
Letters are used as symbols to designate genes
Capital letters are used for dominant genes
Lower case letters are used for recessive genes
Genes always exist in pairs

12. Genetics
A Widows Peak, dominant, would be symbolized with a capital “W”, while no widows peak, recessive, would be symbolized with a lower case “w”.
Father - No Widows Peak - w
Mother - Has a Widows Peak - W

13. Genetics
All organisms have two copies of each gene, one contributed by the father, the other contributed by the mother. Homozygous - Two copies of the same gene ie. BB Heterozygous - Two different genes ie. Bb

14. Genetics
For the widows peak: WW - has a widows peak Homozygous dominant Ww - has a widows peak Heterozygous ww - no widows peak Homozygous recessive Genotype: the set of alleles (the letters: Ww) Phenotype: the physical appearance (widows peak or no widows peak)

15. Genetics
Since Herman has no widows peak, he must be “ww”, since Lilly has a widows peak she could be either “WW” or “Ww” Definitely ww : Homozygous recessive Either Ww : Heterozygous or WW : Homozygous dominant

16. Genetics
We can use a “Punnet Square” to determine what pairs of genes Lilly has
A Punnet Square begins with a box 2 x 2
One gene is called an “allele”
One parents pair is split into alleles on top, the other along the side
Each allele is crossed with the other allele to predict the traits of the offspring
Assume Lilly is heterozygous Ww W w
Assume Herman is homoozygous recessive ww w Ww ww w Ww ww

17. Genetics
Notice that when Lilly is crossed with Herman, we would predict that half the offspring would be “Ww”, the other half would be “ww”
Half “Ww”, Heterozygous, and will have a widows peak
Half “ww”, Homozygous, and will not have a widows peak W w w Ww ww w Ww ww

18. Genetics
Another possibility is that Lilly might be “WW”, homozygous dominant.
Assume Lilly is homozygous dominant WW W W
Assume Herman is homoozygous ww
Notice that all the offspring are heterozygous and will have a widows peak w Ww Ww w Ww Ww

19. Genetics
So which is true? Is Lilly homozygous dominant (WW) or is she heterozygous (Ww)? W w W W w Ww ww w Ww Ww w Ww ww w Ww Ww

20. Genetics
If Lilly were heterozygous, then 1/2 of their offspring should have a widows peak, 1/2 shouldn’t
If Lilly were homozygous, all of their children will have a widows peak W w W W w Ww ww w Ww Ww w Ww ww w Ww Ww

21. Genetics
Recall that Herman and Lilly had another offspring, Marylin. She had no widows peak, therefore, Lilly must be heterozygous.

22. Inheritance Patterns and Mendel's Laws
Terms to Remember
True breeding plant: Mendel created a true breeding line for each variety (trait)
Gene: A segment or portion of DNA that codes for a specific trait.
Trait: A characteristic that can take alternative forms, such as plant height.
Allele: Each variation of a given gene, expressed as either an upper or lower case letter.

23. Inheritance Patterns and Mendel's Laws
Terms to Remember (cont.)
Homozygous: Presence of two of the same alleles for the one genetic trait.
Heterozygous: Presence of two different alleles for one genetic trait.
Dominant allele: Characteristic that, if present on a chromosome, will be expressed in an organism’s phenotype and mask a recessive allele.
Recessive allele: Trait that is only fully expressed when homozygous, rather than paired with a dominant allele
Genotype: An organism’s entire set of alleles.

24. Inheritance Patterns and Mendel's Laws
Terms to Remember (cont.)
Homozygous dominant: A trait that has two dominant alleles.
Homozygous recessive: A trait that has two recessive alleles.
Codominant : Genes that work together to produce a third trait.
Phenotype: Outward expression of an organism’s genotype, typically displayed in physical characteristics.
Testcross: A method of finding the genotype of an organism showing the dominant trait.

25. Inheritance Patterns and Mendel's Laws
Terms to Remember (cont.)
Monohybrid cross: A cross between two organisms for one trait
Dihybrid cross: A cross between two organisms for two traits.
Phenotype ratio: Ratio of possible physical characteristics from a monohybrid or dihybrid cross.
Genotype ratio: Ratio of all possible genotypes from a monohybrid or dihybrid cross.

26. Mendels 1st Law of Genetics: Law of Segregation
Two alleles for each trait must separate when gametes are formed
Therefore a parent only passes on 1 allele for each trait to each offspring
Example:
A parent may have a recessive and a dominate allele for a trait. But only one of these will be passed on to the offspring

27. Mendels 2nd Law of Genetics: Law of Independent Assortment
Genes for different traits are inherited independently of each other
Example:
Color
Shape
Height
Will not affect the
inheritance of each other

28. Mendels 2nd Law of Genetics: Law of Independent Assortment
Genes for different traits are inherited independently of each other
Example:
Color
Shape
Height
Will not affect the
inheritance of each other

29. Inheritance Patterns
Not all alleles are strictly dominant or recessive. There are different ways alleles can interact to create a given phenotype
Type of
Inheritance
How it works
Example
Incomplete
dominance
Allele traits create an intermediate phenotype when an organism is heterozygous
When a plant is heterozygous with a red and white allele, it will produce pink flowers, a blending of the traits
Codominance
Both alleles are expressed fully in heterozygous individuals
In some chickens the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens have a color described as “erminette,” speckled with black and white separate feathers.
Multiple alleles
A situation in which there are more than two possible alleles for a gene.
The ABO blood group in humans, there are three alleles, A, B, O, which produce 6 possible genotypes to form 4 blood types

30. Inheritance Patterns Type of Inheritance How it works Example
Polygenic inheritance
The interaction of multiple genes to form a single characteristic (phenotype)
Height in humans is not limited to being either short or tall but is instead a continuous variation, or wide range, from very short to extremely tall.
Pleiotropy
One gene influences many different characteristics
This is the opposite of polygenic inheritance.
Sex-linked inheritance
The sex chromosomes are X and Y, with most genes carried on the X chromosome. Sex-linked traits are carried on the X chromosome.
Hemophilia is a disease in which blood does not clot properly, only occurs in a female who has two copies of the defective gene (XhXh) . A female with only one copy of the defective gene (XHXh) will not have hemophilia but is a carrier and can pass the defective gene to offspring. Hemophilia occurs in males who inherit only one copy of the defective gene (XhY) . For this reason, sex-linked genetic defects appear much more frequently in males.