1. Patterns of Inheritance
Chapter 9
Winter 2015
Aultman

2. Definitions - 1
Heredity is the transmission of traits from one generation to the next
Genetics is the scientific study of heredity
Gregor Mendel: 1860s; deduced the fundamental principles of genetics
A character is a heritable feature that varies among individuals
Seed color, pod shape, plant height
A trait is a variant of a character (allele)
Yellow or green seeds, wrinkled or inflated pods, dwarf or normal height
Each of the characters Mendel studied occurred in two distinct traits

3. Mendel’s Experiments Garden peas
created purebred varieties of plants
Peas can self-fertilize
crossed two different purebred varieties
Hybrids are the offspring of two different purebred varieties
The parental plants are the P generation.
Their hybrid offspring are the F1 generation.
A cross of the F1 plants forms the F2 generation
A monohybrid cross is a cross between purebred parent plants that differ in only one character

4. Definitions - 2
A gene locus is a specific location of a gene along a chromosome
Alleles alternative versions of genes
For each inherited character, an organism inherits two alleles, one from each parent
An organism is homozygous for that gene if both alleles are identical.
An organism is heterozygous for that gene if the alleles are different.
If two alleles of an inherited pair differ,
then one determines the organism’s appearance and is called the dominant allele and
the other has no noticeable effect on the organism’s appearance and is called the recessive allele.
Dominant alleles determine the plant’s phenotype

5. Mendel’s Law of Segregation
Figure 9.6, page 149
Gametes carry only one allele for each inherited character
The two alleles for a character segregate (separate) from each other during the production of gametes.
This statement is called the law of segregation
Punnett Square
Ratio of offspring characteristics reflects composition of parental traits

6. Mendel’s Law of Independent Assortment
dihybrid cross is the mating of parental varieties differing in two traits
each pair of alleles segregates independently of the other pairs during gamete formation
Offspring have an equal chance of any combination
Figure 9.8 page 150
A testcross is a mating between
an individual of dominant phenotype (but unknown genotype) and
a homozygous recessive individual
Predict the outcome:

7. The Rules of Probability
Mendel’s strong background in mathematics helped him understand patterns of inheritance
The rule of multiplication states that the probability of a compound event is the product of the separate probabilities of the independent events
Figure 9.1. page 154
For a monohybrid cross, each cell of the Punnett square has a 25% probability; each cell – one combination of genes
Homozygous dominant = 25%
Homozygous recessive – 25%
Heterozygous = 25% + 25% = 50%

8. Single-Gene Human Disorders
Many human traits show simple inheritance patterns and are controlled by single genes on autosomes.
Most human genetic disorders are recessive
Individuals who have the recessive allele but appear normal are carriers of the disorder
Cystic fibrosis: recessive allele carried by about one in 31 Americans
Achondroplasia is a form of dwarfism.
The homozygous dominant genotype causes death of the embryo
only heterozygotes have this disorder

9. Incomplete Dominance in Plants and People
F1 hybrids have an appearance between the phenotypes of the two parents
hypercholesterolemia,
heterozygotes have blood cholesterol levels about twice normal, and
homozygotes have about five times the normal amount of blood cholesterol and may have heart attacks as early as age 2.
Polygenic inheritance is the additive effects of two or more genes on a single phenotype

10. Co-Dominance: ABO Blood Groups
The immune system produces blood proteins called antibodies that bind specifically to foreign carbohydrates
A person whose blood has A or B carbohydrates does not make antibodies to them; antibodies are only made to foreign antigens.
If a donor’s blood cells have a carbohydrate (A or B) that is foreign to the recipient, the donated blood cells may be clumped together by the recipient’s blood, potentially killing the recipient
The clumping reaction is the basis of a blood-typing lab test
The human blood type alleles IA and IB are codominant, meaning that both alleles are expressed in heterozygous individuals who have type AB blood

11. Pleiotropy and Sickle-Cell Disease
one gene influences several characters
Sickle-cell disease results in abnormal hemoglobin proteins, and causes disk-shaped red blood cells to deform into a sickle shape with jagged edges

12. Chromosome theory of inheritance
genes are located at specific positions (loci) on chromosomes and
the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns.
chromosomes
undergo segregation and independent assortment during meiosis and
account for Mendel’s laws

13. Linked Genes
are located close together on a chromosome and tend to be inherited together
Thomas Hunt Morgan used the fruit fly Drosophila melanogaster and determined that some genes were linked based on the inheritance patterns of their traits.

14. Genetic Recombination: Crossing Over
Crossing over can
separate linked alleles,
produce gametes with recombinant gametes, and
produce offspring with recombinant phenotypes.
The percentage of recombinant offspring among the total is called the recombination frequency
Linkage Maps
method for mapping the relative gene locations
Figure 9.28, page 164

15. Sex Chromosomes and Sex-Linked Genes
Sex chromosomes influence
Gender: XX or XY
Other traits that are sex-linked
Red-green colorblindness
caused by a malfunction of light-sensitive cells in the eyes
Hemophilia
sex-linked recessive blood-clotting trait that may result in excessive bleeding and death after relatively minor cuts and bruises
Figure 9.31 page 166