1. Mendel’s Laws of Heredity
Why we look the way we look...

2. What is heredity?
The passing on of characteristics (traits) from parents to offspring
Genetics is the study of heredity

3. Mendel used peas... They reproduce sexually
They have two distinct, male and female, sex cells
Their traits are easy to isolate

4. What Did Mendel Find?
He discovered different laws and rules that explain factors affecting heredity.

5. Rule of Unit Factors Each organism has two alleles for each trait
Alleles - different forms of the same gene
Genes - located on chromosomes, they control how an organism develops

6. Rule of Dominance
The trait that is observed in the offspring is the dominant trait (uppercase)
The trait that disappears in the offspring is the recessive trait (lowercase)

7. Heterozygous & Homozygous
Heterozygous - if the two alleles for a trait are different (Aa)
Homozygous - if the two alleles for a trait are the same (AA or aa)

8. Heredity
The two alleles for a trait must separate when the sex cells are formed
A parent randomly passes only one allele for each trait to each offspring

9. Heredity
The genes for different traits are inherited independently of each other.

10. Phenotype & Genotype Phenotype - the way an organism looks
red hair or brown hair
genotype - the gene combination of an organism
AA or Aa or aa

11. Incomplete Dominance
Neither allele is dominant so there is a blending of traits when two different alleles for the same trait occur together. The offspring have a mix of their parents.

12. Incomplete Dominance
Heterozygous individuals = rd phenotype

13. Incomplete Dominance In Four O’ Clocks, if you cross a red
(which is always pure)
with a white (that is
also always pure),
you get a pink
(which is always a hybrid).

14. Incomplete Dominance Cross of two pink flowers
What are the possibilities?
genotype ratio
phenotype ratio

15. Multiple Alleles

16. Multiple Alleles
Some human traits are controlled by a single gene that has two or more alleles.
Three or more forms of a gene that code for a single trait.
In this pattern of inheritance, the genes have more than two alleles controlling them.

17. Multiple Alleles Even though a gene may have
multiple alleles, a person can
only carry two of those alleles.
Chromosomes exist in pairs –
Each chromosome in a pair
only carries one allele for each
gene

18. Multiple Alleles
Blood type in humans is an example of this inheritance pattern.
The four different blood groups:
A, B, O, and AB
Are produced by three different alleles:
A, B, and O

19. Multiple Alleles
Examples of Blood type crosses

20. Blood Type Practice Problems:
A mother is AB and a father is O. Draw the Punnett square.
A B O
What is the probability their offspring will have A type blood? ____
Can they have an offspring with O blood?

21. Polygenic Traits

22. Polygenic Traits
Some human traits show a large number of phenotypes because the traits are controlled by many genes.
The genes act together as a
group to produce a single trait.

23. Polygenic Traits Traits controlled by two or more genes
Show a wide range of phenotypes
The phenotype is produced by the interaction of more than 1 pair of genes.

24. Polygenic Traits In humans, eye color,
skin color, hair color are a few controlled by many genes.

25. Polygenic Traits
Height is controlled by four genes working together.

26. Polygenic Traits
Skin color is controlled by at least three genes, each one containing two different alleles. Various combinations of alleles produce the many skin colors in humans.

27. Genetic Disorders

28. Genetic Disorder
An abnormal condition that a person inherits through genes or
chromosomes.
Genetic disorders are caused
by mutations.
Down syndrome and cystic fibrosis are two examples of genetic disorders.

29. Genetic Recessive Disorders
Many human genetic disorders are caused by recessive genes.
Occur when both parents have the recessive allele for the disorder.
Parents may be heterozygous and have no symptoms and pass the trait onto any offspring.

30. Sex-linked Genetics
Ex. Colorblindness

31. Boy or Girl?
The gender of a baby is determined by genes on chromosomes.
There are 23 pairs of chromosomes in each of our cells. One of pair of
chromosomes are called sex
chromosomes.

32. Boy or Girl? The sex chromosomes determine whether a person is
male or female.
The sex chromosomes are the only pair of chromosomes that do not always match.

33. Females
Two sex chromosomes match XX
Since both chromosomes are X, all eggs carry one X chromosome.

34. Males
Two sex chromosomes don’t match XY, so sperm cells will either carry an X or a Y chromosome.

35. Sex-Linked Genes Some human traits occur more
often in one gender than the other.
Sex-Linked Genes: Genes on the X and Y chromosomes, whose alleles are passed from parent to offspring on sex chromosomes

36. Sex Linked Gene
In females, a dominant allele on one X chromosome will mask a recessive allele on the other X chromosome.
In males, there is no matching allele on the X and Y chromosome. As a result, any allele on the X chromosome will produce the trait in a male who inherits it.

37. Sex-Linked Genes
Because males have only one X chromosome, males are more likely than females to have a sex-linked trait that is controlled by a recessive allele.

38. Sex-Linked Punnett Square
Let C = Normal Vision c = Colorblind
Cross: Normal Male ( ) x Carrier Female ( )

39. Pedigree

40. Pedigree
Pedigree: A chart or “family tree” that tracks which members of a family have a particular trait