Transcript Genotype

Inheritance of Single-Gene
Differences
Transmission genetics – link between meiosis &
Mendel’s postulates
Mendel: father of genetics
I.
II.
A.
B.
III.
Mendel’s Empirical approach
Contrasting characters
Monohybrid cross
A.
B.
C.
IV.
Mendel’s results
Mendel’s First “law” equal segregation
Punnent Square
Dihybrid cross
A.
B.
Mendel’s Second “law” independent assortment
Using the testcross
I. Transmission genetics – link between
meiosis & Mendel’s postulates
The transmission of discrete units
(genes located on chromosomes) from
parent to offspring
Correlation between the behavior of
chromosomes during meiosis and the
transmission of traits
Terminology review
• Genes come in different forms = ALLELES
• Phenotype = expressed form of a character (what an
individual looks like)
• Genotype = specific set of alleles carried by an individual
(the actual genetic composition)
• Homozygous = the alleles of a gene are identical (AA)
• Heterozygous = the alleles of a gene are different (Aa)
• Dominant allele = an allele that expresses its phenotypic
effect even when heterozygous… therefore AA and Aa
have the same phenotype
• Recessive allele = An allele whose phenotypic effect is
notexpressed in a heterozygote… therefore (a) can only
be expressed when the individual is homozygous – (aa).
Genetic Crosses
• Self Cross =
• Haploid Cross = simplest, each gene present in 1
copy only (fungi)
• Diploid Cross = each gene present in 2 copies
II. Gregor Johann Mendel
“Father of Genetics”
Mendel’s success
• Came up with an elegant
model of experimental design
– chose a good “model” organism:
Pisum sativum
– restricted his examination to one
or very few pairs of contrasting
traits in each experiment
– took meticulous notes with
accurate quantitative records
A. Mendel’s Empirical
approach
Mendel’s experiments were
designed to determine the
quantitative relationships from
which laws could be discovered
B. Contrasting characteristics of the garden pea
III. Monohybrid cross
• Hybridization = when two plants
of the same species but with
different characteristics are
crossed (mated) to each other.
• Mono = dealing with one pair of
contrasting characteristics
• P–
• F1 –
• F2 –
A. Mendel’s results
Parental
Round x wrinkled
F1
All
round
F2
5474 round
1850 wrinkled
6022 yellow
2001 green
705 purple 224
white
882 inflated
229 pinched
428 green 152
yellow
651 axial 207
terminal
All long 787 long 277
short
F2 ratio
2.96:1
Yellow x green seeds All
yellow
Purple x white
All
purple
Inflated x pinched
All
inflated
Green x yellow pods All
green
Axial x terminal
All axial
3.01:1
Long x short
2.84:1
3.15:1
2.95:1
2.82:1
3.14:1
Mendel’s explanation
1) the existence of “factors” – particulate theory of
inheritance
2) genes are in pairs,
3) the principle of segregation,
4) gametic content – the F2 3:1 ratio is based on a
1:1 segregation in a heterozygote
5) random fertilization – gametes are brought
together for fertilization in a random manner
B. Mendel’s First Law
• Equal Segregation = The two members of a
gene pair segregate from each other into the
gametes; so half the gametes carry one
member of the pair and the other half of the
gametes carry the other member of the pair.
C. Using Punnett Squares
in Genetic Crosses
Punnett squares
– Considers only genes of interest
– List sperm genotypes across top
– List egg genotypes down side
– Fill in boxes with zygote genotypes
Making a Punnett Square:
Heterozygous X Heterozygous
P
Eggs of Heterozygous Plant
Pollen of
Heterozygous Plant
Genotypes
Frequencies
Phenotypes
Frequencies
PP
1
p
PP
Pp
pP
pp
pP
Pp
2
pp
1
IV. Dihybrid Cross
• Follows the inheritance of two different
traits within the same individual.
A. Mendel’s Second Law
• Independent Assortment = two different
genes will randomly assort their alleles
during gamete formation
F1 cross: GgWw x GgWw
(Hair color) & (Hair length)
Black/Brown
Short/Long
P: Black, short x Brown, long
B. Using the testcross
Pedigree Analysis
How do doctors know if a trait is
inherited?
• They take a “family history”, and
show it in a diagram form known as a
pedigree
Pedigree Symbols
Example 1:
• Grandparents had two
children: a son and a
daughter.
• Their son had the trait in
question. He marries a
woman without the trait.
• One of the son’s four
children (a boy) had the
trait.
• Grandpa has the trait,
Example 2:
grandma doesn’t.
• Of their five children,
one son and two
daughters have trait.
One son and one
daughter don’t have
trait.
• One daughter with trait
marries man without.
Of their five kids, one
son and one daughter
have trait.
Can you tell from a pedigree if a trait
is dominant or recessive?
Could this •trait
dominant?
If it’sbe
dominant,
the affected
daughter would have to be either
DD or Dd.
• If she’s DD or Dd, she would have
had to get a dominant D allele
from a parent.
• But, if were dominant and the
parent had a D allele, the parent
would have the trait, too.
• Therefore, trait is recessive, NOT
dominant; daughter is dd
What are the genotypes?
Dd
dd
Dd
DD
or
Dd
• Parents don’t have the
trait, so they can’t be
dd.
• But, since they each
passed a recessive d
allele to daughter, they
must each be Dd.
• Since the trait is
recessive, daughter
must be dd.
Could this trait be dominant?
• If it’s dominant, the affected
daughter would have to be either
DD or Dd.
• If she’s DD or Dd, she would
have had to get a dominant D
allele from a parent.
• Dad has the trait, so he could
have given her a D allele.
• So, trait is dominant.
What are the genotypes?
dd
Dd
Dd
dd
• Since the trait is dominant,
mom can’t have a D, or she
would show the trait.
• So, mom is dd.
• The son doesn’t have trait, so
he must be dd, too.
• If son is dd, he got one d
from mom; one from dad.
• Dad must be Dd; daughter is
Dd.