Chapter 9 Patterns of Inheritance
Patterns of Inheritance
I. Mendalian genetics
A. Gregor Mendel - European monk
- mid 1800s: people believed traits blended; some thought female contributed more because
ovum > sperm
1. Mendel asked 2 questions:
- Were physical traits blended?
- Do parents contribute equally?
2. Mendel was an excellent scientist
- He spent 2 years studying before selecting peas for experiment
easy to grow Fig 9.5
control fertilization Fig 9.6
easily distinguishable traits Fig 9.7
These plant are easily manipulated
These plants can self-fertilize - Fig 9.5
Mendel carried out some cross-fertilization - Figs 9.6 & 9.7
- repeated experiments
- large sample size
* - statistical analysis
B. Mendels discoveries
1. Principle of segregation Fig 9.8
- Adults have 2 factors for any given trait
- These factors segregate (separate) during gamete formation (sperm, ova)
- Each parent contributes 1 factor to offspring
A monohybrid cross is a cross between parent plants that differ in only one characteristic
An explanation of Mendels results, including a Punnett square
2. Principle of dominance
- Traits are not blended
Dominant trait is expressed
Recessive trait is not
now called alleles alternate forms of the same trait
Ex: pod color:
Dominant = green (G)
Recessive = yellow (g)
Note: Symbol doesnt always start with the first letter of the allele (trait). The first letter of the dominant allele is usually written as a capital letter,
in this instance G for Green.
There are three possible combinations with the G and g:
GG = green - homozygous dominant
Gg = green - heterozygous
gg = yellow - homozygous recessive
genotype = genetic make-up Ex: GG, Gg, gg
phenotype = physical appearance Ex: Green, Yellow
We use Punnett squares to keep track of matings.
We list the genotype in the Punnett square to determine the phenotype of offspring.
Parents (P1) GG X gg truebreeding (between themselves)
Result is F1 generation → all Gg (all heterozygous)
Cross F1 x F1 = F2
Genotype: GG X 1, Gg X 2, gg X 1
Phenotype: green, green X 2, yellow
This is a monohybrid cross → 1 trait
3. Principle of Independent Assortment
- Genes located on different chromosomes segregate independently during meiosis
- Dihybrid cross:
P1 RRYY x rryy
- Get combinations not originally present thus must segregate independently
See Fig 9.10
Q. How do you determine if organism with dominant trait is homozygous or heterozygous?
A. Cross with a homogygous recessive
See Fig 9.12
A testcross is a mating between
An individual of unknown genotype and
A homozygous recessive individual
Why does a testcross work?
D. Rules of probability
- Mendels work all assumed equal numbers of all alleles (not always the case)
- Can be used to predict genotypes without drawing a Punnett square
Ex: Gg x Gg
What is chance of gg ?
In each Gg: ½ are G, and ½ are g
½ X ½ = ¼ Rule of multiplication
What is chance of Gg ?
½ X ½ = ¼ or ½ X ½ = ¼
¼ + ¼ = ½ Rule of Addition independent events
- family tree shows matings and incidence of a given trait
- can be used to determine if a trait is dominant or recessive, and if people are carriers (heterozygotes)
Know Fig 9.15
A family pedigree
Shows the history of a trait in a family
Allows researchers to analyze human traits
F. Human single-gene disorders
1. Table 9.1 shows there is > 1,000 disorders
2. Recessive disorders are more common
Ex. cystic fibrosis
- mating among relatives chance of homozygous recessive disorders ® origin of incest taboos
- note different incidence in different populations
- Table 9.1
3. Dominant disorders dominant allele not necessarily more common nor better than recessive
II. Variations on Mendels principles
A. Incomplete dominance
F1 hybrid has characteristics in between parents
Figs 9.18 and 9.19 KNOW!
Fig 9.19 is an example of incomplete dominance.
In incomplete dominance F1 hybrids have an appearance in between the phenotypes of the two parents
Hypercholesterolemia (Fig 9.19)
Is a human trait that is incompletely dominant
B. Multiple alleles
1. Most genes have more than 2 alleles
Ex: blood types ABO System
Gene I = 3 alleles
IA A carbohydrate on red blood cell
IB B carbohydrate on red blood cell
IO No carbohydrate on red blood cell
IAIA or IAIO ® Type A
IBIB or IBIO ® Type B
IOIO ® Type O
3. Dont confuse co-dominance with incomplete dominance which is blended
Ex: co-dominance ® flower with red and white patches
Ex: incomplete dominance ® snapdragons pink flowers
C. Pleiotropy one gene often affects more than one trait
Ex: Sickle-cell disease Fig 9.21
SS = normal hemoglobin
Ss = 50% normal/50% abnormal
ss = all sickle hemoglobin → disease
Why is this so common in Africans?
malaria parasite → enters RBC → reproduces → malaria
if sickle-cell, triggers sickling, body destroys infected cell → no malaria
D. Polygenic inheritance
- most traits a result of more than 1 gene
Polygenic inheritance is the additive effects of two or more genes on a single phenotype
III. Chromosomal basis of inheritance
Fig 9.23 KNOW!
- Mendel published work in 1866, before mitosis/meiosis figured out
~ 1900 biologists rediscovered his work
(1) Genes are on chromosomes
(2) Behavior of chromosomes during meiosis & fertilization explains inheritance patterns
A. Gene linkage
1. Not all genes are on separate chromosomes (Mendel got lucky!)
Only got AB or ab NO
independent assortment Ch 21 Ch 20
Only got AB or ab
Expect to get AB aB Ab ab
Expect to get
2. Linked genes (on same chromosome) tend to be inherited together, BUT not always. Why?
3. Crossing-over changes linkage.
Fig 9.24a,b,and c
- This principle was very important in early-mid 1900s gene mapping research.
In 1908, British biologists discovered an inheritance pattern inconsistent with Mendelian principles
This inheritance pattern was later explained by linked genes, which are on the same chromosome.
The Process of Science: Are Some Genes Linked?
Using the fruit fly Drosophila melanogaster, Thomas Hunt Morgan determined that some genes were linked based on the inheritance patterns of their traits.
Genetic Recombination: Crossing Over
Two linked genes
Can give rise to four different gamete genotypes.
Can sometimes cross over.
Among the offspring, some with recombinant phenotypes
IV. Sex chromosomes and sex-linked genes - Fig 9.27
A. Any gene on X or Y chromosome
- most are on X-chromosomes, so better called X-linked
- most are recessive
- SRY (Sex-determining Region of Y) is key to testis development ® male
Are designated X and Y
Determine an individuals sex
B. Exhibit characteristic inheritance patterns
Ex: color-blindness Fig 9.30
color blindness = c
Normal color = C
→ normal XCXc → carrier XcXc → color blind Male XCY →
normal XcY →
XCXC → normal
XCXc → carrier
XcXc → color blind
XCY → normal
XcY → color blind
Red-green color blindness
Is characterized by a malfunction of light-sensitive cells in the eyes
- Takes only 1 recessive gene for male to be color blind, but 2 for female. Why?
Other examples: hemophilia, Duchenne MD
Is a blood-clotting disease
(only took 1 gene for disease to appear in males)
C. Y-linked genes
- read Evolution Connection, p. 166