Download Chapter 12

Inheritance Patterns & Human
Genetics
Ch.12
(12-1) Human Males & Females
• Sex chromo.’s (X & Y)
– Male: XY
– Female: XX
• Gametes
– Egg: carry only X
– Sperm: carry X or Y
50% chance
female offspring
50% chance
male offspring
Who Discovered Sex
Chromosomes?
• Thomas Morgan
– Early 1900s
– Columbia University
– Worked w/ fruit flies
Sex Linkage
• Gene on a sex chromo.
– X-linked genes
– Y-linked genes
Sex Linked Traits
• Most sex linked genes found on the X
chromo.
• Genes on the Y chromo. are for male
reproductive organ development
Sex Linked Genetic Problems
• In flies: R = red eyes, r = white eyes
– Gene located on the X chromosome
X
X
Y
X
Example 1
• White eye male mates w/ a red ho/go
dominant female
• XrY x XRXR
Xr
Y
100 % red female
0 % white female
XR
XRXr
XRY
100 % red male
0 % white male
XR
XRXr
XRY
Example 2
• Red eye male mates w/ a red he/go
female
• XRY x XRXr
XR
Y
100 % red female
0 % white female
XR
XRXR
XRY
50 % red male
50 % white male
Xr
XRXr
XrY
Example 3
• White eye male mates w/ a red he/go
female
• XrY x XRXr
50 % red female
50 % white female
50 % red male
50 % white male
Xr
Y
XR
XRYr
XRY
Xr
XrXr
XrY
What would you expect from
crossing 2 he/go flies?
Morgan’s Results
• These characteristics are on the same
chromo., thus they are linked together
during meiosis
Linkage Groups
• Genes located on the same chromo.’s &
therefore inherited together
• Goes against Law of
Indep. Assortment
How do linked genes get
“unlinked”?
• Crossing over
• Frequency of
crossing over b/w
certain genes is
used to make a
chromosome map
Chromosome Map
• Linear sequence of genes on a chromo.
• 1 map unit = 1% chance of crossing over
• Farther apart = greater chance of genes to
be separated
Which 2 genes have the highest
probability of crossing over?
The lowest?
a
A
b
B
Highest: A & C
Lowest: A & B
c
C
Mutation
• Change in DNA
• Entire chromo. or a single nucleotide
• Can lead to genetic disorders or be
beneficial
Mutation Types
• Germ-cell mutation: occurs in gametes
– Does not affect the organism, but can be
passed on
• Somatic-cell mutation: occurs in body cells
– Does affect organism & is not passed on
• Lethal mutation: causes death, often
before birth
Chromosome Mutations
• Change in the structure of a chromo.
– Deletion/addition
– Inversion: segment breaks off & reattaches in
reverse
– Translocation: segment breaks off &
reattaches to another chromo.
– Nondisjunction: failure of chromo.’s to
separate during meiosis
Nondisjunction
Gene Mutations
• Point mutation: substitution at 1 point in
DNA
• Changes 1 codon
• Frame-shift mutation: cause the
misreading of all codons after the mutation
Substitution
Deletion
(12-2) Pedigree
• Diagram showing how a trait is inherited
over several generations
Half-filled = carrier
Human Patterns of Inheritance
1.
2.
3.
4.
5.
6.
Single allele trait
Multiple allele trait
Polygenic trait
X-linked trait
Sex-influenced trait
Nondisjunction
Single Allele Trait
• Trait controlled by a single allele of a gene
• Normal dom.-rec.
• Ex:
– Huntington’s Disease (autosomal dom.)
– Cystic Fibrosis (autosomal rec.)
Multiple Allele Trait
• 3 or more alleles of the same gene code
for a single trait
• Ex: ABO Blood Groups
– IA = type A (dom.)
– IB = type B (dom.)
– i = type O (rec.)
Blood Type Genotypes
• What are the genotypes for the following blood
types?
–A
• IAIA or IAi
–B
• IBIB or IBi
–O
• ii
– AB
• IAIB
Blood Type Problems
• A mother gives birth to a type O child. The
mother is type A blood. The 2 potential
fathers are type A (father 1) and type AB
(father 2). Who’s the daddy?
Father 1 (Type A)
IA
i
IA
i
A
A
IA
A
i
O
IA
IB
A
A
AB
B
Polygenic Trait
• Trait that’s controlled by 2 or more genes
– Most human characteristics
• Range of phenotypes
– Influenced by environment too (complex
characters – environ. & genes)
• Exs:
– Skin color
– Eye color
– Human height
X-Linked Trait
• Trait controlled by a gene on the X
chromo.
• Exs:
– Colorblindness (rec.)
– Muscular dystrophy (rec.)
– Hemophilia (rec.)
Sex-Influenced Traits
• Presence of sex hormones influences
expression of these traits
• Ex:
– Pattern baldness
Nondisjunction
• Failure of chromo.’s to separate during
meiosis resulting in 1 gamete w/ too many
chromo.’s & 1 w/ too few
Nondisjunction (cont.)
• Trisomy: cell w/ 3 copies of a chromo.
• Monosomy: cell w/ 1 copy of a chromo.
• Ex:
– Down Syndrome (Tri-21)
– Klinefelter’s syndrome (XXY)
– Turner’s Syndrome (XO)
Detecting Disorders
• Before pregnancy:
– Genetic screening
– Genetic counseling
• During pregnancy:
– Amniocentesis
– Chorinonic villi sampling
• After birth:
– Genetic screening