2. Chapter 13: Patterns in Inherited Traits
Unit 5
Chapter 13: Patterns in Inherited Traits

3. Gregor Mendel Austrian monk, Father of Genetics
Genetics: the study of heredity
Heredity: the passing of traits from parent to offspring (INHERITANCE)
Mendel used pea plants to study heredity
Pea plants self-fertilize, so Mendel cross-pollinated peas by hand to observe traits of their offspring
Started with pea plants that “bred true” for a trait (stays the same every generation), then cross-fertilized pea plants with different traits
Offspring appeared in predictable patterns, he concluded that hereditary information is passed in discrete units

4. Gregor Mendel Parent Generation (P): 1st line of crosses
First Generation(F1): offspring of the parent generation
F2 Generation: second cross, using the F1 offspring

5. Inheritance in Modern Terms
Individuals share certain traits because their chromosomes carry the same genes
The DNA sequence of each gene occurs at a specific location on a particular chromosome

6. Sexual Reproduction Alleles: Different forms of the same gene
For example: height
Tall or short
We inherit one allele for a gene from each parent
Offspring of sexual reproducers inherit new combinations of parental alleles which produces new traits
Genetic variation!!!!

7. Inheritance in Modern Terms
Alleles are either dominant or recessive
Dominant alleles mask recessive alleles, the recessive allele is there, just hidden
A dominant allele is represented by italic capital letters (A)
A recessive allele is represented by italic lowercase letters (a)

8. Inheritance in Modern Times
Individuals only need to inherit one dominant allele to see that trait – DOMINANCE
TWO recessive alleles must be inherited to see that trait
Homozygous: An individual carrying identical alleles for a gene [true or pure] – AA or aa
Heterozygous: An individual carrying two different alleles of a gene [hybrids] – Aa
Parents of these offspring breed true for a trait
We see the dominant trait, the organism carries the recessive trait

9. Inheritance in Modern Terms
Genotype: the particular set of alleles that an individual carries [heterozygous or homozygous]
Phenotype: the observable traits, such as flower color
Genotype gives rise to phenotype

10. Inheritance in Modern Terms
EXAMPLE –
If detached earlobes are dominant, attached earlobes are recessive
TT – homozygous dominant
Tt – heterozygous, since the dominant allele is present, it will show
tt – homozygous recessive
GENOTYPE (HETEROZYGOUS)
T t =
Tall
ALLELE
ALLELE
PHENOTYPE

11. Mendel’s Law of Segregation
Two alleles for a trait separate during meiosis and are reunited during fertilization
Each gamete will have a different allele

12. Mendel’s Law of Independent Assortment
When homologous chromosomes separate during meiosis, they are randomly assorted in to any nucleus
Gene pairs on one chromosome get sorted into gametes independently of gene pairs on other chromosomes
Every person with brown hair doesn’t have brown eyes
Some genes are inherited together (LINKED) because the genes are very close to each other on the chromosome.
People with red hair are also fair-skinned.
Punnett squares can be used to predict inheritance

14. Punnett Square
Punnett Square: grid used to predict the genotypic and phenotypic outcome of a cross
Monohybrid: crossing one trait at a time
Dihybrid: crossing two traits
Testcross: Breeding experiments used to determine genotype

15. Punnett Square
An individual shows a dominant trait (RR or Rr) and is crossed with one that is homozygous recessive (rr)
What is the unknown genotype?
If all offspring have the dominant trait = homozygous dominant
If any offspring have the recessive trait = heterozygous

16. Dominant Trait (T) is tongue roller
Recessive Trait (t) is non-tongue roller
2 heterozygous (Tt) parents T t
Genotypic
Homo Dominant = 1
Heterozygous = 2
Homo Recessive = 1
1:2:1 TT Tt T
Tongue roller
Tongue roller Tt tt t
Phenotypic
Tongue Roller = 3
Non-tongue Roller = 1
3:1
Non-tongue roller
Tongue roller
Probability of having a child who could roll their tongue? [3 of 4 = 75%]
A child who could not? [1 of 4 = 25%]

17. Dihybrid Cross
Individuals identically heterozygous for alleles of two genes (dihybrids) are crossed, and the traits of the offspring are observed

18. Complex Inheritance
Simple Dominance: dominant allele fully masks the expression of a recessive one
Other patterns of inheritance are not so simple, and cannot be explained by Mendel’s law of inheritance
Incomplete dominance
Codominance
Epistasis
Pleiotropy

19. Complex Inheritance
Incomplete Dominance: One allele is not fully dominant over another
heterozygous phenotype is a blend of homozygous phenotypes
Codominance: heterozygous genotype expresses BOTH alleles

20. Complex Inheritance
Multiple Alleles: gene for which three or more alleles exist in a population
Example: an ABO gene for blood type
This doesn’t mean an organism has more than 2 alleles for a trait, just that more than 2 exist in the population

21. Complex Inheritance The A and the B allele are codominant when paired
Genotype AB = type AB
The O allele is recessive when paired with either A or B
Genotype AA or AO = type A
Genotype BB or BO = type B
Genotype OO = type O

22. Complex Inheritance
Epistasis: a trait is influenced by the products of multiple genes
Fur color in dogs – A dominant allele (B) specifies black fur, recessive (b) specifies brown fur, but a dominant allele of a different gene (E) causes color to be deposited in fur and the recessive (e) reduces color
E and B allele = black fur, E and bb = brown, ee = yellow fur regardless of B or b alleles

23. Complex Inheritance
Pleiotropy: A gene whose product influences multiple traits, also called polygenic traits
Mutations in pleiotropic genes are associated with complex genetic disorders
Sickle-cell anaemia, Cystic fibrosis, Marfan syndrome

24. Epigenetics
Potentially inheritable mechanisms that alter genes without changing the DNA sequence
These can cause environmental adaptation faster than evolution, and the changes can be reversed
Chemicals and diet can cause changes, but traumatic experiences in our past, or even our recent ancestors’ pasts, can leave molecular scars on our DNA

25. Environmental Factors
Epigenetic research is revealing that environment can influence phenotype
Toxic agents, Diet and exercise, Sunlight and water, Temperature, Medications
Conditions can cause a gene to shut down or turn on
Twins have identical genes, scientists conclude that twins with different phenotypes are influenced by the environment
“Nature versus nurture”