Chapter 9  Patterns of Inheritance


Patterns of Inheritance

I. Mendalian genetics

A. Gregor Mendel - European monk

- mid 1800’s: 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

Fig 9.5

•         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. Mendel’s 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

Fig 9.8.a

•         An explanation of Mendel’s results, including a Punnett square

Fig 9.8.b

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 doesn’t 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)


G           G












–       Result is F1 generation → all Gg (all heterozygous)


–       Cross F1 x F1 = F2


G           g














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


C. Testcross

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

- Mendel’s 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

E. Pedigree

- “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 Mendel’s 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

IAIB             ® Type AB

IOIO             ® Type O



3. Don’t 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

Fig 9.21


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

Ex: height

skin color

Fig 9.22


•         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



Ch 21


Ch 20



¯                 ¯

Expect to get

AB     aB

Ab     ab






Fig 9.23


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.


Fig 9.24


Genetic Recombination: Crossing Over

–       Two linked genes

•         Can give rise to four different gamete genotypes.

•         Can sometimes cross over.


Fig 9.25


Fig 9.26

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


•         Sex chromosomes

•         Are designated X and Y

•         Determine an individual’s sex

B. Exhibit characteristic inheritance patterns

Ex: color-blindness     Fig 9.30

color blindness = c

Normal color = C



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

Fig 9.30


- Takes only 1 recessive gene for male to be color blind, but 2 for female. Why?


Other examples: hemophilia, Duchenne MD


Fig 9.31


•         Hemophilia

•         Is a blood-clotting disease

(only took 1 gene for disease to appear in males)


C. Y-linked genes

- read Evolution Connection, p. 166