1. Patterns of Inheritance
Chapter 9

2. Gregor Mendel Deduced the fundamental principles of genetics
Cross-fertilization

4. What’s with the Peas? Little spontaneous variation between generations
Can self-fertilize
Easy to control pollination
Possessed several easily observable traits
Pea form
Pea color
Flower location
Flower color
Stem size

5. What do we get?? Genetic cross P generation F1 generation
Bb x Bb
P generation
F1 generation
F2 generation

6. Monohybrid Cross
Cross between parent plants that differ in only one characteristic
Mendel developed four hypotheses from the monohybrid cross:
There are alternative forms of genes
Alleles
For each characteristic, an organism inherits two alleles
One from each parent
Alleles can be dominant or recessive
Gametes carry only one allele for each inherited characteristic

7. Mendel’s Laws
Genes
Set of instructions that determine characteristics of an organism
Segments of nucleic acid that specifies a trait
Found at designated place on chromosomes
Locus
Not all copies of a gene are identical

8. Mendel’s Laws
Alternative forms of a gene lead to the alternative form of a trait
Alleles
way of identifying the two members of a gene pair which produce opposite contrasting phenotypes
Chromosomes that are homologous are members of a pair and carry genes for the same traits in the same order

9. Genes v. alleles Genes Alleles Basic instruction Sequence of DNA
General
Hair color
Alleles
Variations of that instruction
Specifics
Brown hair

10. Alternative alleles of genes are located on homologous chromosomes

11. Genotype verse Phenotype
the alleles an individual receives at fertilization
Homozygous
an organism has two identical alleles at a gene locus
Heterozygous
an organism has two different alleles at a gene locus
Phenotype
the physical appearance of the individual

12. The journey from DNA to phenotype

13. Mendel Proposes a Theory
genetic traits are assigned a letter symbol referring to their more common form
Dominant traits are capitalized while a lower-case letter is reserved for the recessive trait
P signifies purple
p signifies white

14. Describing Genotypes Homozygous Dominant
when both alleles are dominant BB
Homozygous Recessive
when both alleles are recessive bb
Heterozygous
when one allele is dominant and one is recessive Bb

15. Punnet Square…..
Genetic cross determines arrangement

16. Variations on Mendel’s Laws
Incomplete dominance
Multiple allelism & codominance
Pleiotropy
Polygenic inheritance
Environmental factors

17. Why Some Traits Don’t Show Mendelian Inheritance
Incomplete dominance
Alleles have combined (equal) effect on phenotype of heterozygote
Phenotype is intermediate

18. Multiple Allelism: existence of more than 2 alleles of gene
Example: Blood type (A, B, O)
Remember: Each person still only has 2 alleles for that trait, but more than 2 exist

19. Multiple Allelism: Blood typing
ABO Blood Type in Humans exhibits multiple allelism
Phenotype Genotype
O OO
A AA or AO
B BB or BO
AB AB *
How many ALLELES are there?
3 ( A, B, O)
How many Phenotypes are there?
4 (A, B, AB, O)
How many Genotypes are there? 6

20. Question: If a woman with blood type O marries a man with blood type B, can they have a child with blood type A?
Phenotype Genotype
O OO
A AA or AO
B BB or BO
AB AB*
No. The mother’s genotype must be OO and the father’s either BB or BO. Their child will either be type B (BO) or type O (OO)

21. Why Some Traits Don’t Show Mendelian Inheritance
Codominance
A gene may have more than two alleles in a population
in heterozygotes, there is not a dominant allele
Both alleles are expressed
Relationship between the A & B alleles in blood typing

22. Why Some Traits Don’t Show Mendelian Inheritance
Pleiotropic effects
Allele that has more than one effect on a phenotype
these effects are characteristic of many inherited disorders
Sickle-cell anemia
Must be homozygous for sickle cell allele

23. Why Some Traits Don’t Show Mendelian Inheritance
Continuous variation
Characters can show a range of small differences when multiple genes act jointly to influence a character
Polygenic

24. Phenotypes are not always a direct translation of genotype
Phenotypes may also be influenced by the environment
Examples?
skin color influenced by sun
height/weight influenced by nutrition
animal coat influenced by climate

25. Remember…..
P = G + E

26. Sex-linked Traits Female XX Male XY
Genes located on the X or Y chromosome are sex-linked
X and Y chromosomes are not homologous, they contain different genes

27. Sex-linked traits Sex chromosomes Sex-linked genes
Are designated X and Y
Determine an individual’s sex
Influence the inheritance of certain traits
Sex-linked genes
Are any genes located on a sex chromosome

28. Sex-Linked Traits
Females (XX) have 2 copies of each gene on the X chromosome
Males (XY) have only 1 copy of each gene on the X chromosome
Females can show a dominant condition if present on 1 or both X chromosomes
Females can only show a recessive condition if present on both X chromosomes
Males ALWAYS show X-linked alleles, regardless of dominance

29. Sex-Linked Disorders in Humans
number of human conditions result from sex-linked (X-linked) genes
Red-green color blindness
characterized by a malfunction of light-sensitive cells in the eyes

30. Question…..
Will a mother that is colorblind automatically have a son that is colorblind?

31. Mutations
Changes to the nucleotide sequence of the genetic material of an organism
Can be caused by:
copying errors in the genetic material during cell division
exposure to UV light or chemical mutagens
Viruses
can be induced by the organism itself
Create variety within gene pool
Less favorable verse more favorable

32. Human Heredity
To study human heredity, scientists examine crosses that have already been made
Pedigree
Determine whether a trait is sex-linked or autosomal and whether the trait’s phenotype is dominant or recessive

33. Huntington’s disease is a dominant genetic disorder

34. Recessive Disorders Most human genetic disorders are recessive
Individuals can be carriers of these diseases