1. Patterns of Inheritance

2. Why study genetics? A. Human genetic disorders
B. Gene therapy, the future of medicine
1. Locate the problem gene on a chromosome
2. Isolate the gene to study its protein structure and function
3. Design a biomolecule to correct for the defect

3. Gregor Mendel “Father” of modern genetics
First developed rules now used to predict inheritance
Chose to study the pea
Easily manipulated in breeding experiments

4. Mendel’s 7 Pea Plant Traits
Plant height
Flower color
Flower position
Pod color
Pod shape
Seed color
Seed shape

5. sperm-containing pollen)
stamens (male, produce
sperm-containing pollen)
Figure: 11-03b
Title:
The seeds and flowers of the edible pea.
Caption:
In the intact pea flower, the lower petals form a container enclosing the reproductive structures. Pollen normally cannot enter the flower from outside, so peas normally self-fertilize.
intact pea flower
carpel (female
contains eggs)
flower dissected to show
reproductive structures

6. purple parent all P sperm and eggs white parent all p sperm and eggs
Figure: 11-03UN05
Title:
Gametes from a homozygous parent.
Caption:
white parent
all p sperm and eggs

7. The foundation of genetics
Mendel's discoveries
1. Paired alleles control the inheritance of traits
a. Homozygous—two of the same allele for a particular gene
b. Heterozygous—two different alleles for a particular gene

8. chromosome 1 from tomato pair of homologous chromosomes
Both chromosomes carry the same allele
of the gene at this locus. The organism is
homozygous at this locus.
This locus contains another gene for which
the organism is homozygous.
Figure: 11-01
Title:
The relationships among genes, alleles, and chromosomes.
Caption:
Homologues carry the same gene loci.
Each chromosome carries a different
allele of this gene, so the organism is
heterozygous at this location.

9. Mendel’s First Law Law of segregation
Alleles are randomly donated from parents to offspring
Key points of theory
All traits determined by two factors
Factors (alleles) segregate during the formation of gametes
Two factors – joined together in offspring

10. homozygous parent gametes
Figure: 11-03UN03
Title:
A homozygous parent produces only one type of gamete.
Caption:

11. pollen Parental generation (P) pollen cross-fertilize true breeding,
purple-flowered
plant
true breeding,
white-flowered
plant
Figure: 11-03UN01
Title:
Parental generation and 1st generation offspring.
Caption:
First-generation
offspring (F1)
all purple-flowered
plants

12. self-fertilize 3/4 purple 1/4 white
First-generation
offspring (F1)
self-fertilize
Figure: 11-03UN02
Title:
First-generation offspring and second-generation offspring.
Caption:
Second-generation
offspring (F2)
3/4 purple
1/4 white

13. The foundation of genetics
Interaction of the two alleles results in expression of only one
Dominant—allele that expresses itself
Recessive—an allele that is masked (or not visibly apparent) if a dominant allele is present

14. heterozygous parent gametes
Figure: 11-03UN04
Title:
A heterozygous parent produces two types of gametes.
Caption:

15. F1 offspring sperm eggs or
Figure: 11-03UN06
Title:
Hybrid offspring.
Caption:

16. gametes from F1 plants F2 offspring sperm eggs
Figure: 11-03UN07
Title:
Self-fertilization of a Pp pea.
Caption:

17. Figure: 11-04
Title:
The Punnett square method.
Caption:
The Punnett square method allows you to predict both genotypes and phenotypes of specific crosses; here we use it for a cross between plants that are heterozygous for a single trait, flower color. Assign letters to the different alleles; use uppercase for dominant and lowercase for recessive. Determine all genetically different gametes that can be produced by the male and female parents. Draw the Punnett square, with the columns labeled with the egg genotypes and the rows labeled with the sperm genotypes. (We have included the fractions of these genotypes with each label.) Fill in the genotype of the offspring in each box by combining the genotype of sperm in its row with the genotype of the egg in its column. (We have placed the fractions in each box.) Count the number of offspring with each genotype. (Note that Pp is the same as pP.) Convert the number of offspring of each genotype to a fraction of the total number of offspring. In this example, out of four fertilizations, only one is predicted to produce the pp genotype, so 1/4 of the total number of offspring produced by this cross is predicted to be white. To determine phenotypic fractions, add the fractions of genotypes that would produce a given phenotype. For example, purple flowers are produced by 1/4PP + 1/4Pp + 1/4pP, for a total of 3/4 of the offspring.

18. Mendel’s Interpretation
Assumed that each form of trait
Was controlled by a hereditary factor
Dominant traits (R) mask presence of recessive (r) alleles
Phenotype – apparent traits in individuals
Genotype – the genetic complement to phenotype

19. The foundation of genetics
Law of segregation
a. Paired alleles of a gene separate during meiosis, resulting in gametes that contain only a single allele for each gene present in an organism
b. Monohybrid cross is a cross of two parents differing by a single genetic trait
1) Genotype is the genetic makeup of an individual (the alleles present for particular genes)
2) Phenotype is the expressed trait or characteristic (may or may not be visible) resulting from gene expression

20. Punnett Squares
British geneticist, Reginald C. Punnett developed in early 20th century
Predict possible genotypes

21. The foundation of genetics
Law of independent assortment
Alleles can sometimes act independently
a. Pairs of alleles that control different traits segregate independently of each other during meiosis (given that the alleles are on separate chromosomes) or
b. Each homologous pair of chromosomes aligns independently of any other homologous pair during metaphase I of meiosis

22. The foundation of genetics
Dihybrid cross is a cross of two parents differing by two genetic traits

23. Genetic linkage and chromosome maps
Genes on the same chromosome tend to be inherited together
Degree of linkage between two genes is a function of the distance between the genes on the chromosome

24. replicated homologues
pairs of alleles on two pairs of
homologous chromosomes in
a diploid cell
chromosomes
replicated
replicated homologues
pair during metaphase
of meiosis I, orienting
like this
or like this
meiosis I
Figure: 11-07
Title:
Independent assortment of alleles.
Caption:
Chromosome movements during meiosis produce independent assortment of alleles, as shown here for two genes. Each possible combination is equally likely, producing gametes in the proportions 1/4 SY, 1/4 sy, 1/4 sY, and 1/4 Sy.
meiosis II SY sy Sy sY
independent assortment produce four equally likely allele combinations during meiosis

25. Genetic linkage and chromosome maps
Crossing over (during meiosis) rearranges alleles of different genes that were previously linked, creating recombinants
Recombinants are combinations of genes not found in either of the parents
Genes that are closer together on a chromosome are more tightly linked than genes farther apart on a chromosome

26. Genetic linkage and chromosome maps
Linkage maps estimate relative distances between genes on a chromosome by observing the percentage of recombinant offspring resulting from experimental crosses
Locating a gene on a chromosome is the first step to being able to clone the gene

27. Why aren’t members of same species identical?
Almost every organism is the result of thousands of genes working together
There may be many different alleles for a trait in a population
A combination of traits gives an organism a competitive advantage
Mutations are another source of variety

28. Does Mendel’s Law Always Apply?
Lethality
If particular combination of alleles is deadly to new embryo
The embryo dies & phenotype is not represented in next generation at all
One gene can influence two or more traits

29. Does Mendel’s Law Always Apply?
Lethality
Dominant lethal allele kills its recipient

30. Variations on the Mendelian theme
A. Incomplete dominance
B. Polygenic inheritance
C. Gene interactions
D. Multiple effects of a single gene
Pleiotropy—one gene having multiple effects

31. P: F1: F2: Figure: 11-10 Title: Incomplete dominance. Caption:
The inheritance of flower color in snapdragons is an example of incomplete dominance. (In such cases, we use capital letters for both alleles, here R and R'.) Heterozygtes (RR') have pink flowers, whereas the homozygotes are red (RR) or white (R'R').

32. Does Mendel’s Law Always Apply?
Two or more genes can influence a single trait
Human height is controlled by many traits
Termed “Polygenic”

33. A Polygenic Trait

34. mother father Figure: 11-11 Title: Human eye color. Caption:
At least two genes, each with two incompletely dominant alleles, determine human eye color. A brown-eyed man and a brown-eyed woman, each heterozygous for both genes, could have children with five different eye colors, ranging from light blue (no dominant alleles) through light brown (two dominant alleles) to almost black (all four dominant alleles).

35. Human Genetic Disorders
Most genetic diseases are caused by recessive alleles
Sickle Cell Anemia
Cystic Fibrosis

36. (a) Figure: 11-15a Title: Sickle-cell anemia. Caption:
(a) Normal red blood cells are disc-shaped with indented centers. (b) The red blood cells of a person with sickle-cell anemia become sickled.
(a)

37. Autosomal Recessive Traits
Most affected children are the children of unaffected parents
Risk of an affected child from mating of heterozygotes is 25%
Expressed equally in males and females

39. Cystic fibrosis Phenotype
Phenotype
production of thick secretions – often block the ducts from which they are extruded
often malnourished and many respiratory infections
eventually cysts form in the pancreas and it degenerates
- individuals are often infertile  

41. What is the likelihood that a child will inherit cystic fibrosis?
Carriers are heterozygotes
Populations: White 1:22 carriers
Black 1:17000
Asian 1: 90,000
For whites 1/22 X 1/22 = 1 in 484 chance that a carrier will marry a carrier
Chance for a baby with cystic fibrosis
1/22 X 1/22 X 1/4 = 1 in 1936
What is the chance that 2 carriers will have a baby with CF?

42. at least one parent is affected
Autosomal Dominant Traits
at least one parent is affected
- is a 50:50 chance of an affected offspring
males and female offspring have equal chance of being afflicted
- two affected parents can have a normal child
- homozygous dominant often more severe phenotype than heterozygotes

44. Defect - defective gene on chromosome 15
Marfan Syndrome
- inheritance - autosomal dominant
Phenotype - tall and thin, with long extremities, deficiencies in the skeletal system, eyes and cardiovascular system.
Defect - defective gene on chromosome 15
- affected gene produces abnormal fibrillin
- end result - abnormal connective tissue
- major problem - aorta rupture
1 in occurrence
- both male and females afflicted
- 25% of cases occur in families with no previous history
Reason - gene undergoes a high mutation rate

45. Normal Marfan Syndrome

46. Human Genetic Disorders
A few human genetic disorders are caused by dominant alleles
1. Normally die before reproducing —usually no phenotypic carriers
2. Exception is Huntington's disease

47. Sex determination and sex linked genes
FEMALE PARENT
EGGS
MALE PARENT
Figure: 11-09
Title:
Sex determination in mammals.
Caption:
Male offspring receive their Y chromosome from the father; female offspring receive the father’s X chromosome (labeled Xm). Both male and female offspring receive an X chromosome (either X1 or X2) from the mother. S P E R M
FEMALE OFFSPRING
MALE OFFSPRING

48. Human Genetic Disorders
Sex-linked disorders
1. Baldness
2. Red-green color blindness
3. Hemophilia

49. Human Genetic Disorders
Disorders resulting from nondisjunction during meiosis
1. Abnormal number of sex chromosomes
a. Turner's syndrome (XO): sterile, short female; lacks Barr bodies
b. Trisomy X (XXX): fertile female; no detectable defects; decreased intelligence
c. Klinefelter's syndrome (XXY): mixed secondary sex characteristics; sterile male
d. XYY males: decreased intelligence; increased height, increased predisposition for violence

50. Human Genetic Disorders
Abnormal numbers of autosomal chromosomes
Trisomy 21
(Down syndrome): changes increase with increasing age of mother or possibly father

51. Figure: 11-17
Title:
Down syndrome frequency increases with maternal age.
Caption:
The shape of the blue line shows that a low percentage of 20-year-old mothers give birth to a child with Down syndrome, but the percentage begins to grow before age 30 and increases dramatically after age 35.