Barron`s Ch 7 ppt Heredity

Chapter 7: Heredity
Overview of this chapter:
Mendelian laws
Manipulation of Mendelian laws
o Backcross/Testcross
o Monohybrid/Dihybrid Cross
Beyond Mendelian Inheritance
o Incomplete Dominance/Codominance
o Multiple Alleles
Gene Interactions
o Pleiotropy/Epistasis
Linked Genes, Cross-over, Linkage Mapping
Barr Body
o Gene/Chromosomal Mutations,
Gregor Mendel
The father of modern genetics
is Gregor Mendel
- Austrian monk, in 1850s
- Bred garden peas in order to
study patterns of inheritance
- Theory of genetics is one of
particulate inheritance
- Mendelian Laws:
Law of Dominance
Law of Segregation
Law of Independent
Law of Dominance
- When two
organisms, each
homozygous (pure)
for two opposing
traits are crossed,
the offspring will
be hybird (carry
two different
alleles; Gg) but will
exhibit only the
dominant trait (G)
- Mendel’s first law:
Law of dominance
- The trait that
remains hidden is
known as the
recessive trait (g)
Law of
- During formation of
gametes, the two
traits carried by each
parent separate
- Best exemplified by
monohybrid cross
- Trait that’s not evident
in either parent
appears in F1
Thank you Jackie for the amazing paint skills.
Backcross or Testcross
- Way to determine genotype of an
individual plant or animal showing
only dominant trait
- Ex.) individual (B/_) is crossed with
homozygous recessive (b/b)
- If individual is in fact homozygous
dominant, all offspring will be B/b
and show dominant trait. - No
offspring showing recessive trait.
- If individual is hybrid (B/b) - one
half of offspring show recessive
- Therefore if any offspring show
recessive trait, parent must be
Law of Independent Assortment
- Applies when cross is carried out between two
individuals hybrid for two or more traits are not on
the same chromosome. - called dihybrid cross.
- Law states that during gamete formation, alleles for
one trait (e.g. height), segregate independently from
the alleles of a gene for another trait (e.g. seed
- Only factor that determines how these alleles segregate
(assort) is how the homologous pairs line up in
metaphase of meiosis I, which is random.
- However, if the genes are linked then they will not
assort independently.
The Dihybrid Cross
- Cross between two F1 plants is
called a dihybrid cross
- Cross between individuals that
are hybrid for two different
individuals that are hybrid for
two different traits (e.g.
height and seed colour)
This cross can produce four different types of gametes: TY, Ty, tY,
ty (dihybrid cross)
- Many different genotypes possible in the resulting F2 generation.
- Phenotype ratio of the dihybrid cross, 9:3:3:1, 9 tall, yellow; 3
tall, green; 3 short, yellow; and 1 short, green. F2: 9/16 tall,
yellow; 3/16 tall, green; 3/16 short, yellow; 1/16 short, green.
Incomplete Codominance Dominance both traits show
dominance is
characterized by
- Neither trait is
dominant, thus the
convention for
Multiple Alleles
writing the genes
-When there are more
uses a different
than two allelic forms
convention: all
of a gene
capital letters.
Gene Interactions
- Pleiotropy: Ability of single
gene to affect organism in
several ways. - e.g.
autosomal recessive
disease; cystic fibrosis.
- Epistasis - two separate genes control one trait but one masks expression of other gene
- Gene that masks the expression of the other gene
is epistatic to the gene that's masked
Polygenic: genus that vary
along a continuum, e.g. skin
colour, height, hair colour.
Genes and the Environment
- Environment can alter expression of
- Development of intelligence is result of
interaction of genetic predisposition and
- Genus from previous generations
“learned” of a way to “better” do
Linked Genes
- genes on same chromosomes
- Tend to be inherited together and do not assort
independently (unless separated by cross-over)
-Of 46 human chromosomes
- 44 are autosome and 2 are sex chromosomes
-traits carried on X chromosome are sex-linked
-females (XX) inherit two copies of sex-linked
-males (XY) inherit only one copy of sex-linked
-mutated X-linked gene is: X-can lead to sex-lniked traits such as colour
blindness, hemophilia and Duchenne muscular
Important facts about sex-linked
- All daughters of affected fathers are carriers!
-Remember: Sex-linked traits are located on X chromosomes.
- Son has 50% percent chance of inheriting sex-linked trait from
carrier mother
- No “carrier" state for X-linked traits in males.
- If male has gene, he will express it!
- Uncommon for female to have recessive sex-linked condition (for it
to happen, she must inherit mutant gene from both parents)
Cross-over and linkage mapping
- The farther two genes on chromosomes - more likely
to be separated by cross-over during meiosis.
- When cross-over/recombination occur, one can see
chiasmata (physical bridge built around point of
- Result of cross-over = recombination
- Major source of variation!
- Map unit: distance within which recombination
occurs 1 percent of the time.
- Tells of order of linked gene on chromosome (pretty
useful for mapping genome)
Ex) 1) Genes A, B, D are linked
2) Cross-over or recombination frequencies for B and
D is 5%, B and A is 30%, D and A is 25%
3) Draw linkage map from this (name BDA or ADB)
Important Tools to Know
Recombination Frequency:
Number of recombinants
------------------------------------- x 100
Total number of offspring
X-Inactivation - Barr body
- In development of embryo in
female mammal, one of X
chromosome is inactivated
in every somatic cell
- Inactivation occurs randomly
- Results in genetic mosaic
- Some cells have one X inactivated, some cells have other X
inactivated, therefore all cells of female mammals not
- Inactivated chromosome condenses into dark spot of
chromatin - Barr body
Remember: All female body cells have one Barr Body.
Normal male cells have none.
Mutations: any changes in the genome
- Two types: gene mutations/chromosomal mutations
- Chromosomal mutations can be seen with karyotype
-Chromosomal aberrations: Deletion, Inversion,
Translocation, Polyploidy, (Duplication)
Some examples....
Genetic Disorder
Pattern of Inheritance
Cystic fibrosis
Autosomal recessive
Huntington's disease
Autosomal dominant
Sex-linked recessive
Chromosomal Disorder
Pattern of Inheritance
Down syndrome
47 chromosomes due to trisomy 21
Turner's syndrome
XO 45 chromosomes due to a missing sex
Nondisjunction: an error
that sometimes occurs
during meiosis in
which homologous
chromosome fail to
separate as they
Aneuploidy: Abnormal number of chromosomes
Triploid: Extra set of chromosomes (3n)
Tetraploid: Organism with (4n)
Polyploid: Organism with extra sets of chromosomes
- Common in plants
- Results in abnormally large size plants
- Some cases: responsible for new species
Triploid: Extra set of chromosomes (3n)
Strawberries (8n)
Genomic Imprinting +
Extranuclear Genes
- Two inheritance patterns that are exceptions to Mendelian inheritance
- genomic imprinting + extranuclear genes
Genomic imprinting: a variation in phenotype depending on whether a trait is
inherited from mother or father.
- Occurs during gamete formation - caused by silencing of particular allele by
methylation of DNA, therefore zygote expresses only one allele of imprinting
- Imprint carried to all body cells and passed through generations
- Imprinted gene located on autosomes
Extranuclear gene are located on mitochondria and chloroplasts
- DNA is small, circular, carry small number of genes
- Linked to rare/severe inherited genes in humans
- Mutations in these genes cause weakness/deterioration in muscles
- Mitochondrial DNA is inherited only from mother because father’s mitochondria
does not enter egg during formation.
Wrapping it
up with a
cool picture!

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