Download Chapter 6 Genetics and Inheritance - Linn

6/2/2015
Incomplete Dominance
Chapter 6
Genetics and Inheritance
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Sometimes there is not one clear dominant allele
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In a heterozygous individual, both alleles are
expressed
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Phenotype is a blend of both traits
Lecture 2: Genetics and Patterns of
Inheritance (continued)
Incomplete Dominance
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Both red (RR) and white (rr
(rr)) are dominant
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Heterozygous (Rr
(Rr)) = pink
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Use a Punnett square to predict the ratio of
red:pink:white offspring if 2 pink snapdragons
are crossed
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Example in humans: hair
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Both curly (CC) and straight (SS) are dominant
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Heterozygous (CS) = wavy
Use a Punnett square to predict the probability
of a child with wavy hair from a father with wavy
hair and a mother with straight hair
Incomplete Dominance
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Genotype?
Maternal alleles
S
S
S
Incomplete Dominance
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Genotype?
C
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Example: snapdragon color
Paternal alleles
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Incomplete Dominance
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Codominance
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Incomplete vs. Codominance
Commonly seen when more than 2 alleles exist
for the same gene
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Both dominant alleles are expressed at once
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Not a blend of the 2 traits – both distinct
traits can be seen at the same time
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Incomplete dominance
Dominant
Dominant
and codominance are
NOT the same thing!!
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Incomplete dominance:
phenotype is a blend
of the two traits
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Codominance:: both
Codominance
traits are seen at
the same time
Codominance
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Human example: A, B, O blood types
Both type A and type B are dominant
•
•
•
(IA
and
IB)
Make different glycoproteins on the
membrane of red blood cells
Before the days of DNA testing, blood type was
used to settle paternity suits
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Type O is recessive
•
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Chaplin Paternity Case
Doesn’t always work though
Charlie Chaplin was involved in such a case in
1942 with actress Joan Barry
Makes no such glycoprotein due to a
frameshift mutation that produces a STOP
codon
If IA and IB are both present, both will be
expressed
Chaplin Paternity Case
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Charlie Chaplin’s blood type: AB
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Joan Barry’s blood type: O
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Child’s blood type: O
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Use a Punnett square to determine whether
Charlie Chaplin could have been the child’s
father
Chaplin Paternity Case
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Charlie Chaplin’s blood type: AB
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•
•
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Only possible genotype:
Joan Berry’s blood type: O
Only possible genotype:
Child’s blood type: O
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Only possible genotype:
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Chaplin Paternity Case
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Charlie Chaplin’s blood type: AB
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•
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All of our examples of inheritance patterns have
focused on single genes
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Humans have 25,000 genes!
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Genes on the same chromosome are inherited
together
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Genes on different chromosomes are inherited
separately
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If we consider just 2 chromosomes, how many
different gametes could be produced by
meiosis?
IAIB
Joan Berry’s blood type: O
•
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Only possible genotype:
Independent Assortment
Only possible genotype: ii
Child’s blood type: O
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Only possible genotype: ii
Independent Assortment
Independent Assortment
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4 possible gametes produced
Consider 2 pea plants
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Heterozygous for 2 traits on 2 different
chromosomes
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Can create a dihybrid Punnett square to
examine all offspring possibilities
Independent Assortment
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Trait 1: seed shape
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Dominant = spherical (S)
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Recessive = dented (s)
Trait 2: seed color
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Dominant = yellow (Y)
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Recessive = green (y)
Independent Assortment
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Both plants to be crossed: SsYy
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Phenotype?
What are the possible gamete combinations?
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(Each will get one allele of each gene)
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Recombination
Recombination
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Sometimes it appears that 2 traits on the same
chromosome sort independently
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DNA is
replicated
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Recall the process of crossing over during
meiosis…
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4 copies of
each
chromatid
form a tetrad
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Portions of
homologous
chromosomes
are swapped
Recombination
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This process is called recombination
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Remember: 25,000 genes in human genome
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Between recombination and independent
assortment there are essentially infinite
genetic combinations for the gametes of any
individual
Genetics of Gender
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Chromosomal basis of gender discovered in 1905
by Nettie Stevens
Genetics of Gender
Genetics of Gender
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Homogametic: sex chromosomes are the same
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Heterogametic: sex chromosomes are not the
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Males = XY
same
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Females = XX
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Humans (and most animals)
Homogametic sex is usually (but not always)
default
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Birds, some insects
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Females = ZW
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Males = ZZ
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Genetics of Gender
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Chromosomal basis for gender discovered by
studying 2 disorders:
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Turner’s Syndrome (XO)
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Kleinfelter’s Syndrome (XXY
(XXY))
Both caused by nonnon-disjunction of sex
chromosomes
(Sperm
could also
be XY)
Turner’s Syndrome (XO)
Kleinfelter’s Syndrome (XXY)
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Only nonnon-fatal complete monosomy
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Not a true trisomy,
trisomy, but 47 chomosomes present
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Incidence: 1:2000
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Incidence 1:500 – 1:1000
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Short stature
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Highly varied
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Non--functioning ovaries
Non
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Sometimes obvious at puberty
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Webbed neck
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Sometimes only discovered
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Swelling of hands and feet
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Skeletal abnormalities
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High blood pressure
(sometimes) less muscle,
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Heart defects
poor beard growth,
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Kidney problems
tall stature, breast development
when seeking help for infertility
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Mosaicism
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Chromosome nonnon-disjunction takes place in
mitosis during embryonic development
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Some cells will have normal chromosome counts
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Some cells will have 45 or 47 chromosomes
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Extreme case: hermaphroditism
Less testosterone =
Mosaicism
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Also occurs normally with X chromosomes
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Called X silencing
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One X chromosome is silenced in every cell of
the female body
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Which X is silenced depends on the cell
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Usually not noticeable
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Example: calico cat
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SexSex
-Linked Inheritance
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When one gender is affected by a genetic
disorder more than the other
In humans = most often males
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X chromosome = large
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Y chromosome = small
SexSex
-Linked Inheritance
Fragile X Syndrome
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Many genes on X chromosome
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X-linked dominant disorder
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Males are essentially monosomatic for these
genes
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Variably expressed
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Results from >200 CGG
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Whatever allele is present will be expressed
repeats that lead to DNA
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Examples: hemophilia, muscular dystrophy, color
blindness, many lysosomal storage diseases,
Fragile X syndrome
methylation and silencing of FMR1
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Causes physical and
intellectual changes
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Nearly all children with
Fragile X meet criteria for
autism diagnosis
Colorblindness
X-Linked Recessive Pedigree
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Genes for color vision are on X chromosome
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No corresponding gene on Y chromosome
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Normal color vision is dominant, so males are
most often affected
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Consider the following pedigree for a family with
colorblindness
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Consider…
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Assign genotypes to each individual
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How many carriers are there?
XCY-
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Male with normal color vision:
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Female carrier with normal color vision: XCxC
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Females are affected by XX-linked disorders
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Far less common
What is the probability that their child will be
colorblind?
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