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Extending Mendelian
Genetics
Chapter 7 page 186
A. Chromosomes and Phenotype
1. 2 copies of each autosomal gene affect phenotype
a. Inherit 1 set of chromosomes from each parent
b. Homologous chromosomes could have same gene but
different alleles
c. Gene expression often related to whether the gene is on an
autosome or sex chromosome
d. Disorders caused by recessive alleles
1) Only homozygous recessive individuals express the
disorder
2) Carrier – does not show symptoms but can pass on the
disease
3) Parents are typically heterozygous for the trait
4) Ex – Cystic fibrosis
Punnett Square for Cystic Fibrosis
e. Disorders caused by dominant alleles
1) Less common than recessive disorders
2) At least one parent will have the disorder
3) Huntington’s disease
2. Sex-linked traits
a. Sex-linked genes
1) Genes located
on the sex chromosomes
2) XX genotype = female
and XY genotype = male
3) Females can only give an X,
males can give an X or Y
4) Genes on X and Y chromosomes
are sex-linked
5) X chromosome carries nearly
1100 genes, but the Y
chromosome has less than 250
6) More traits are controlled by
the X chromosome
b. Expression of sex-linked genes
1) Sex linked traits are more common in males
a) Males only have 1 X chromosome
b) One recessive trait on an X chromosome gives an
expressed trait
2) Females
a) X chromosome inactivation – 1 X chromosome is
randomly turned off
Female
b) Causes a “patch work” of
different genes in cells
c) Female calico and
tortoiseshell cats
Male
B. Complex Patterns of Inheritance
1. Incomplete Dominance
a. Heterozygous phenotype is b/w the two homozygous
phenotypes
b. Neither is completely dominant or recessive
c. Beta fish and snap-dragon
Incomplete dominance
Green (B1B1)
Steel Blue (B2B2)
Royal Blue (B1B2)
2. Codominance
a. Both traits are expressed equally
b. Red flowers x white flowers = red and white patches
c. Blood type
1) A and B alleles are codominant
2) O allele is recessive
3) A and B blood means you make proteins called
antigens
Human Blood Types
3. Polygenic Traits
a. Traits produced by two or more
genes
b. Skin, eye, and fur color
c. Green dominant over blue yet recessive
to brown
d. Epistatic gene
1) Interferes with all other
gene expression
2) Can cause albinism
4. Environmental influence
a. Gender of turtles before hatching
1) Warm temperatures = females
2) Cooler temperatures = males
b. Genotypes may express different phenotypes
1) Hair changes color in summer from the sun
2) Artic fox has white fur in winter and brown in summer
c. Identical twins raised in different environments
29˚ C = 84.2˚ F
Arctic Fox
Summer
Winter
C. Human Genetics and Pedigrees
1. Studying genetics of simple organisms helps us understand
our genetics
2. Genetics are the same in all sexually reproducing organisms
3. Inheritance of many human traits is very complex
4. Females can carry sex-linked disorders
a. Males only have 1 X chromosome
1) They express every X-linked trait (dominant or recessive)
2) Nothing to mask recessive traits
3) Males can’t be carriers of sex-linked traits only females
b. Expression of disorder depends on:
1) Which parent carries the allele
2) Sex of the child
c. Ex: Hemophilia and color blindness
5. Pedigrees
a. Chart which traces geno/phenotypes in a family to
determine if people are carriers
b. Used by genetic counselors to determine probability of
parents having children with genetic disorders
c. Determining autosomal trait or sex-linked:
1) Sex-linked – appear more often with males
2) Autosomal – appear equally b/w both males and
females
d. Key
1) Boxes – males
2) Circles – females
3) Shaded – person expresses trait
4) Half shaded – person is a carrier
5) Line through it – person has died
6) Connecting lines – shows mate or children
Colorblindness
Colorblindness Test
#3
#4
#5
#6
#7
#8
Fig. 4.4 pg 204
6. Karyotypes
a. Genome – all DNA in a cell
b. Karyotype – picture of all chromosomes in a cell
c. Used to detect large changes like #, size, or shape of
chromosomes
d. Human Genome Project
1) International effort
2) Tries to map, sequence, and ID all genes in human
genome
Human Karyotype
Is this karyotype
from a male or
female?