1. Chapter 12: Patterns of Inheritance
Fig. 12.2
Chapter 12: Patterns of Inheritance
Chapter 13: Chromosomes, Mapping, and the Meiosis-Inheritance Connection

2. Patterns of Inheritance
Chapter 12

3. Early Ideas of Heredity
Before the 20th century, 2 concepts were the basis for ideas about heredity:
heredity occurs within species
traits are transmitted directly from parent to offspring
Led to the belief that inheritance is a matter of blending traits from the parents.

4. Early Ideas of Heredity
Botanists in the 18th and 19th centuries produced hybrid plants.
When the hybrids were crossed with each other, some of the offspring resembled the original strains, rather than the hybrid strains.
This evidence contradicted the idea that traits are directly passed from parent to offspring.

5. Fig

6. Early Ideas of Heredity
Gregor Mendel
Studied the pea plant:
other research showed that pea hybrids could be produced
many pea varieties were available
peas are small plants and easy to grow
peas can self-fertilize or be cross-fertilized

7. Early Ideas of Heredity
Gregor Mendel’s experimental method:
Produce true-breeding strains for each trait he was studying
Cross-fertilize true-breeding strains having alternate forms of a trait
perform reciprocal crosses as well
Allow the hybrid offspring to self-fertilize and count the number of offspring showing each form of the trait

9. Fig

10. Fig

11. Monohybrid Crosses
Monohybrid cross: a cross to study only 2 variations of a single trait
Mendel produced true-breeding pea strains for 7 different traits
each trait had 2 alternate forms (variations)
flower color, pea color, pea shape...
Mendel cross-fertilized the 2 true-breeding strains for each trait

12. Monohybrid Crosses F1 generation (1st filial generation):
offspring produced by crossing 2 true- breeding strains
For every trait Mendel studied, all F1 plants resembled only 1 parent
No plants with characteristics intermediate between the 2 parents were produced

14. Monohybrid Crosses
F2 generation (2nd filial generation): the offspring resulting from the self-fertilization of F1 plants

17. Monohybrid Crosses
F2 generation (2nd filial generation): the offspring resulting from the self-fertilization of F1 plants
Dominant traits: the form of each trait most commonly expressed in the F1 plants
Recessive traits: the form of the trait not seen in the F1 plants; hidden traits

18. Monohybrid Crosses
F2 plants exhibited both forms of the trait in a very specific pattern:
¾ plants with the dominant form
¼ plant with the recessive form
The dominant to recessive ratio was 3 : 1.
Mendel discovered the ratio is actually:
1 true-breeding dominant plant
2 not-true-breeding dominant plants
1 true-breeding recessive plant

21. Monohybrid Crosses
Gene: information for a trait passed from parent to offspring
Alleles: alternate forms of a gene
Homozygous: having 2 of the same allele
Heterozygous: having 2 different alleles

22. Monohybrid Crosses Genotype: total set of alleles of an individual
PP = homozygous dominant
Pp = heterozygous
pp = homozygous recessive
Phenotype: outward appearance of an individual
physical manifestation of genotype

23. Monohybrid Crosses Principle of Segregation:
Two alleles for a gene segregate during gamete formation
homologous chromosome separation during meiosis
Alleles are re-paired at random, one from each parent, during fertilization.

26. Monohybrid Crosses Some human traits are controlled by a single gene.
some of these exhibit dominant inheritance
some of these exhibit recessive inheritance
Carriers are heterozygous
Pedigree analysis is used to track inheritance patterns in families.

29. Dihybrid Crosses
Dihybrid cross: examination of 2 separate traits in a single cross
for example: RR YY x rryy
The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait.

31. Dihybrid Crosses
The F2 generation is produced by crossing members of the F1 generation with each other or allowing self-fertilization of the F1.
for example RrYy x RrYy
The F2 generation shows all four possible phenotypes in a set ratio:
9 : 3 : 3 : 1

33. Dihybrid Crosses Principle of Independent Assortment:
In a dihybrid cross, the alleles of each gene assort independently.

34. Probability – Predicting Results
Rule of addition: the probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities.
When crossing Pp x Pp, the probability of producing Pp offspring is:
probability of obtaining Pp (1/4), PLUS probability of obtaining pP (1/4)
¼ + ¼ = ½

35. Probability – Predicting Results
Rule of multiplication: the probability of 2 independent events occurring simultaneously is the PRODUCT of their individual probabilities.
When crossing Rr Yy x RrYy, the probability of obtaining rr yy offspring is:
probability of obtaiing rr = ¼
probability of obtaining yy = ¼
probability of rr yy = ¼ x ¼ = 1/16

36. Testcross
Testcross: a cross used to determine the genotype of an individual with dominant phenotype
cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp)
the phenotypic ratios among offspring are different, depending on the genotype of the unknown parent

38. Extensions to Mendel Mendel’s model of inheritance assumes:
each trait is controlled by a single gene
each gene has only 2 alleles
there is a clear dominant-recessive relationship between the alleles
Most genes do not meet these criteria.
Other types of inheritance:
Polygenic, Pleiotropy, Incomplete dominance, codominance

39. Extensions to Mendel
Polygenic inheritance occurs when multiple genes are involved in controlling the phenotype of a trait.
The phenotype is an accumulation of contributions by multiple genes.
These traits show continuous variation and are referred to as quantitative traits.
For example – human height

41. Extensions to Mendel
Pleiotropy refers to an allele which has more than one effect on the phenotype.
This can be seen in human diseases such as cystic fibrosis or sickle cell anemia.
In these diseases, multiple symptoms can be traced back to one defective allele.

42. Extensions to Mendel
Incomplete dominance: the heterozygote is intermediate in phenotype between the 2 homozygotes.
Codominance: the heterozygote shows some aspect of the phenotypes of both homozygotes.

44. Extensions to Mendel The human ABO blood group system:
-multiple alleles: there are 3 alleles of the I gene (IA, IB, and i)
-codominance: IA and IB are dominant to i but codominant to each other

46. Extensions to Mendel
The expression of some genes can be influenced by the environment.
for example: coat color in Himalayan rabbits and Siamese cats
an allele produces an enzyme that allows pigment production only at temperatures below 30oC

47. Extensions to Mendel

48. Extensions to Mendel
The products of some genes interact with each other and influence the phenotype of the individual.
Epistasis: one gene can interfere with the expression of another gene

50. Chromosomes, Mapping, and the Meiosis-Inheritance Connection
Chapter 13

51. Chromosome Theory Chromosomal theory of inheritance
developed in 1902 by Walter Sutton
proposed that genes are present on chromosomes
based on observations that homologous chromosomes pair with each other during meiosis
supporting evidence was provided by work with fruit flies

52. Chromosome Theory T.H. Morgan isolated a mutant white-eyed Drosophila
Red-eyed female X white-eyed male gave a F1 generation of all red eye
Morgan concluded that red eyes are dominant

53. Fig. 13.1

54. Chromosome Theory Morgan crossed F1 females X F1 males
F2 generation contained red and white- eyed flies but all white-eyed flies were male
Testcross of a F1 female with a white-eyed male showed the viability of white-eyed females
Morgan concluded that the eye color gene is linked to the X chromosome

57. Sex Chromosomes
Sex determination in Drosophila is based on the number of X chromosomes
2 X chromosomes = female
1 X and 1 Y chromosome = male
Sex determination in humans is based on the presence of a Y chromosome
having a Y chromosome (XY) = male

58. Sex Chromosomes
Sex-linked traits: traits controlled by genes present on the X chromosome
Sex-linked traits show inheritance patterns different than those of genes on autosomes.
In many organisms, the Y chromosome is greatly reduced or inactive.
Genes on the X chromosome are present in only 1 copy in males

60. Sex Chromosomes
Dosage compensation ensures an equal expression of genes from the sex chromosomes even though females have 2 X chromosomes and males have only 1.
In each female cell, 1 X chromosome is inactivated and is highly condensed into a Barr body.
Females heterozygous for genes on the X chromosome are genetic mosaics.
Phenotype depends on which X chromosome inactivated

62. Chromosome Theory Exceptions
Mitochondria and chloroplasts contain genes.
Traits controlled by these genes do not follow the chromosomal theory of inheritance
Genes from mitochondria and chloroplasts are often passed to the offspring by only one parent

63. Chromosome Theory Exceptions
Maternal inheritance: uniparental (one- parent) inheritance from the mother
The mitochondria in a zygote are from the egg cell; no mitochondria come from the sperm during fertilization
In plants, the chloroplasts are often inherited from the mother, although this is species dependent

64. Genetic Mapping
The science of determining the location of a gene on a chromosome
Early geneticists realized that they could obtain information about the distance between genes on a chromosome.
This is genetic mapping
Mapping is based on genetic recombination (crossing over) between genes.

67. Genetic Mapping To determine the distance between genes:
dihybrid organisms are testcrossed
offspring resembling the dihybrid parent result from homologues that were not involved in the crossover
offspring resulting from a crossover are called recombinant progeny

68. Genetic Mapping
The distance between genes is proportional to the frequency of recombination events.
recombination recombinant progeny
frequency total progeny
1% recombination = 1 map unit (m.u.)
1 map unit = 1 centimorgan (cM) =

72. Genetic Mapping
Determining the order of genes can be done with a three-point testcross
The frequency of double crossovers is the product of the probabilities of each individual crossover
Therefore, the classes of offspring with the lowest numbers represent the double crossovers and allow the gene order to be determined

75. Human Genetic Disorders
Some human genetic disorders are caused by altered proteins:
the altered protein is encoded by a mutated DNA sequence
the altered protein does not function correctly, causing a change to the phenotype
the protein can be altered at only a single amino acid (e.g. sickle cell anemia)

78. Fig

79. Human Genetic Disorders
Some genetic disorders are caused by a change in the number of chromosomes.
Nondisjunction during meiosis can create gametes having one too many or one too few chromosomes
Fertilization of these gametes creates trisomic or monosomic individuals
Down syndrome is trisomy of chromosome 21

81. Human Genetic Disorders
Nondisjunction of sex chromosomes can result in:
XXX triple-X females
XXY males (Klinefelter syndrome)
XO females (Turner syndrome)
OY nonviable zygotes
XYY males (Jacob syndrome)

82. Sex Determination in Humans
The sex chromosomes, X and Y, are a homologous pair:
this pair is unique because X and Y carry different sets of genes
the Y chromosome has genes that determine maleness
the X chromosome has a variety of genes on it
XX = female; XY = male
X_ = female; _Y = does not survive
XXY = male 82

83. Sex Determination in Humans
Genotype: X__
Sex: Female
Phenotype: Turner’s Syndrome
short stature (less than 5’)
‘webbed’ neck and other physical characteristics
infertility 83

84. Sex Determination in Humans
Genotype: XXX
Sex: Female
Phenotype: ‘Super-females’, metafemales
tall stature
longer legs and torso
may have learning disabilities or emotionally underdeveloped
commonly labeled as ‘trouble makers’ in school 84

85. Sex Determination in Humans
Genotype: XYY
Sex: Male
Phenotype: ‘Super-males’
produce higher levels of testosterone
may be taller than average
no known significant abnormalities 85

86. Sex Determination in Humans
Genotype: XXY or XXXY
Sex: Male
Phenotype: Klinefelter Syndrome
produce very little testosterone
taller and more overweight than average
may have feminine characteristics
sterile or nearly sterile
most have normal cognitive abilities
can be treated with testosterone early in life 86

87. Table

88. Table

90. Human Genetic Disorders
Genomic imprinting occurs when the phenotype exhibited by a particular allele depends on which parent contributed the allele to the offspring
A specific partial deletion of chromosome 15 results in:
Prader-Willi syndrome if the chromosome is from the father
Angelman syndrome if it’s from the mother

91. Human Genetic Disorders
Genetic counseling can use pedigree analysis to determine the probability of genetic disorders in the offspring.
Some genetic disorders can be diagnosed during pregnancy:
amniocentesis collects fetal cells from the amniotic fluid for examination
chorionic villi sampling collects cells from the placenta for examination