1. Patterns of Gene Inheritance
Chapter 23

2. Genetics is the study of genes
What is a gene?
A. A factor that controls a heritable characteristic
B. Something on a chromosome
C. Information stored in a segment of DNA
D. Something that encodes a protein

3. Gregor Mendel

4. LOTS O’ DEFINITIONS
Gene---A piece of DNA that encodes a particular trait. EX a gene for eye-color
Allele—an alternate form of a gene. EX—allele for blue eyes and allele for brown eyes
Phenotype—the physical expression of a gene or allele. Ex. Blue eyes
Genotype—the genetic composition of an individual.

5. MORE DEFINITIONS
Locus—the location of a gene on a chromosome. Plural=loci
Dominant allele—expressed whether alone or in pairs. Symbolized by a capital letter. EX Brown eye allele=B
Recessive allele—expressed only in the absence of a dominant allele. Symbolized by lowercase. EX blue eye allele=b
What is the maximum # of alleles a diploid individual can have at any given locus?

6. STILL MORE DEFINITIONS!!!
Genotype—the genetic composition of an individual
Homozygous----containing a pair of the same alleles. Can be
Homozygous recessive—two recessive alleles EX. bb, or
Homozygous dominant– two dominant alleles EX BB
Heterozygous—containing two different alleles. EX Bb

7. Law of Segregation
Each individual has two factors (called genes today) for each trait.
Factors segregate during gametogenesis. Ploidy level of gametes?
Fertilization gives each new individual two factors again.
Ploidy level after fertilization?

8. Gene locus
Defs

9. Example--Widow’s Peak

10. Gametogenesis Homologous pairs separate during meiosis. When?
 a gamete has only one allele from each pair of alleles.
 If the primary spermatocyte is genotype Ww, the sperm cell would contain either a W or a w, but not both.
Ww represents the genotype of an individual.
Gametes are represented by W or w.

11. One-Trait Crosses
GENOTYPE?
GENOTYPE?
GENOTYPE?

12. Punnett Square EXAMPLE 1—A Monohybrid Cross

13. Monohybrid cross

14. The One-Trait Testcross
QUESTION: How can we determine the genotype of a dominant phenotype?
The One-Trait Testcross
Testcross: cross the dominant phenotype (unknown genotype) with the recessive phenotype (known genotype).

15. The Law of Independent Assortment
Law of Segregation involves 1 character. What about 2 (or more) characters?
Each pair of alleles segregates independently of the other pairs
All possible combinations of alleles can occur in the gametes
EX: two pairs of alleles segregate independently of each other.
When will different alleles not sort independently?

16. Independent Assortment
Mother
Father N E n e OR E n e N
Alignment of Homologs at Metaphase I
Replication N e E n E n e N e n E N e n E N
Telophase II

17. Segregation and Independent Assortment

18. MEIOSIS & GENETIC VARIATION
Independent Assortment
Diploid organisms can produce 2n diff. gametes
Ea. homologous pair can orient in two different ways
Humans: 223 = 8,388,608
(est. 8.4 million)
Each homologous pair can orient in either of two ways

19. HYPOTHETICAL EXAMPLE OF INDEPENDENT ASSORTMENT
Eye color
Hair color
Gene for brown eyes
Gene for blue eyes r
Gene for black hair
Gene for red hair

20. INDEPENDENT ASSORTMENT
During meiosis I, tetrads can line up 2n different ways. OR
Meiosis I & II
Brown eyes Black hair
Blue eyes Red hair
Blue eyes Black hair
Brown eyes Red hair

21. Linked Genes and Independent Assortment
Mother
Father E n e N L l
Replication N e E n E n e N N E n e OR L L l l
Alignment of Homologs at Metaphase I l L

22. Dihybrid cross
What are the possible genotypes of this individuals gametes?
Probabilities

23. Two-trait Testcross-How?

24. Genetic Disorders
Pedigree charts- show patterns of inheritance

25. Autosomal Dominant Disorders
Autosomes---Non-sex chromosomes
Dominant allele
when under what genotype(s)will an individual show the disorder?

26. Examples: Autosomal Dominant Disorder
Neurofibromatosis
Small benign tumors,
Gene on chromosome 17.
Huntington Disease
Progressive degeneration of the nervous system
Normal until middle age
No treatment
Gene has been isolated (chromosome 4), genetic testing can diagnose

27. Huntington disease

28. Autosomal recessive pedigree chart
Autosomes---Non-sex chromosomes
Recessive allele
when under what genotype(s)will an individual show the disorder?

29. Autosomal Recessive Disorders
Tay-Sachs Disease
Common among United States Jews of central and eastern European descent.
Death by the age of three or four.
Defective enzyme in lysosomes.
Cystic Fibrosis
Most common lethal genetic disorder among Caucasians
Defective chloride ion transport protein
Osmotic imbalance results in thick mucous in lungs and pancreatic ducts

30. Cystic fibrosis therapy

31. Phenylketonuria (PKU)
allele on chromosome 12.
lack an enzyme needed for metabolism of phenylalanine (an amino acid)
Urine test diagnostic.
Brain damage unless controlled by diet.

32. Polygenic Inheritance
Polygenic (Quantitative) Traits
Governed by more than one gene pair.
Several genes determine the phenotype.
Produce bell-shaped curve.
EX: Skin color

33. EX 2 Polygenic Inheritance :

34. Polygenic Disorders
Examples: cancer, schizophrenia, hypertension, diabetes, etc
several genes involved
also environmental influences.

35. Multiple Allelic Traits
Often more than two alleles exist for a particular gene locus.
Maximum # of alleles at any given locus?
Each individual inherits only two alleles for these genes!!!
EX: Human Blood type

36. Inheritance of blood type
Details

37. Incompletely Dominant Traits
Codominance--both alleles equally expressed in a heterozygote. EX ABO Blood type
Incomplete dominance--heterozygote shows intermediate phenotype, representing a blending of traits. EX Wavy hair in Caucasians
Phenotypic ratio ?
1 : 2 : 1.

38. Incomplete dominance

39. Example 2-Incomplete Dominance

40. Sickle-Cell Disease Caused by incompletely dominant alleles.
Single nucleotide mutation causes abnormal hemoglobin.
HbA = normal hemoglobin
HbS = sickled condition.
Genotype of normal?
Genotype of sickled homozygote?
Genotype of intermediate phenotype?

41. Sickle-Cell Disease

42. Molecular Explanation-Dominance
Let’s Get Small!!!
Molecular Explanation-Dominance
Simple Dominance
One-half the amount of gene product sufficient for phenotype
Incomplete Dominance
Recessive allele not expressed in heterozygote
Homozygous dominant –2 doses, full phenotype
Heterozygote—1 dose, reduced phenotype
Codominant
Both alleles expressed, combined phenotype

43. Everything’s Relative
Fruit Flies--- The workhorse of eukaryotic genetics.

44. WHAT’S HAPPENING??

45. SUMMARY Physical Basis of Genes Lots o’ Definitions Law of Segregation
Law of Independent Assortment
One-Trait Crosses
Two-Trait Crosses
Genetic disorders
Polygenic Traits
Incomplete Dominance/Codominance

46. REMEMBER!!! Relate genetics to meiosis
For genetics problems—first try to figure the genotype(s) of the gametes