Transcript Genetics: A Monk a Pea and a Fly

Genetics: A Monk a
Pea and a Fly
Mendel and His Peas
• Born in Czech
Republic in 1822
• Studied math in
Vienna
• Was in charge of the
monastery garden
• Conducted controlled
breeding between
true-breeding pea
plants to study
inheritance
Garden Peas : Tasty
and Educational
• Easy to grow
• Fast growing
• Number of easily
recognized traits
• Lots of offspring
(each seed)
• Easy to control
crossing
Mendel’s Crosses
Dominant and
Recessive Traits
• Noticed traits in the F1 were not a blend of
the parents but, instead favored one
parent
• Trait that shows up the F1 generation is
Dominant the trait that is masked is
Recessive
• Today : Traits chemical factors = Genes
Forms of a trait = Alleles
The F1 Cross
• Mendel then crossed the F1 plants and the
recessive trait reappeared!
• What happened?
*Segregation : Alleles on homologous
chromosomes separate when sex cells
are produced
Think about the logic here
A dominant trait is one
that …
1. Is always present in the genes
2. Is seen even when the recessive allele is
present
3. Is more common in the population
4. Is better than the recessive trait
How would I know that
I was growing a truebreeding plant
0%
43%
29%
29%
1. It would reproduce asexually
2. It always made the same type of peas
3. When crossed with a similar plant the
offspring would be just like the parents
4. When crossed with a different plant the
offspring have traits of both plants
The law of segregation
states that ….
65%
30%
5%
0%
1. Alleles on homologous chromosomes
separate
2. Homologous chromosomes do not
influence each other’s inheritance
3. Alleles on sister chromatids separate
4. Some chromosomes get different alleles
Genotypes and
Phenotypes
• Genotype : expression of alleles present
Dominant alleles : Capital first letter of
the dominant trait
Recessive alleles : Lower case script of
the first letter of the dominant trait
Homozygote : Two of the same allele
Heterozygote : Two different alleles
• Phenotype : Actual visible trait
• Example : Flower color
Purple (P) is dominant to white (p)
Cross a pure breeding plant with purple
flowers with a pure breeding plant with
white flowers.
1) Determine genotypes
2) Segregate alleles
3) Perform cross
Probability and
Genetics
• Chance of an event occurring : probability
• Does not indicate something will happen
only the likelihood that it can happen
• Allele combinations follow rules of
probability
• Probabilities in genetics can be
determined using simple math or Punnett
squares
Mr. Curry’s Never Miss,
Easy, Solves Every
Type of Genetics
Problem Method
1.
2.
3.
4.
5.
6.
Define alleles
Define phenotypes associated with genotypes
Write out cross
Produce gametes
Perform cross (Punnett square)
Determine genotypic and phenotypic ratios of
offspring
7. Answer the problem
Typical Monohybrid
• Homozygous Dominant X Homozygous
Recessive
P=
F1=
F2=
Two Traits at Once :
Dihybrid Cross
• How do you determine inheritance for two
traits at once?
- Dihybrid Cross
• Just remember : Segregation and
Independent Assortment (For now, these
genes exist on separate chromosomes)
• In the fruit fly Drosophila melanogaster, vestigial wings
and hairy body are produced by two recessive alleles
carried on different chromosomes. The normal alleles,
long wings and hairless body, are dominant. If a
vestigial-winged, hairy male is crossed with a female
homozygous for
•
both of the normal traits, what would be the
phenotypes and genotypes of their progeny? If the
•
F1 generation was allowed to mate randomly among
themselves, what phenotypes and genotypes would be
expected among the F2’s, and in what proportions?
• In some breeds of dogs, a dominant allele controls the
characteristic of barking while trailing. In
•
these dogs, another, independent gene controls ear
shape, erect ears being dominant over floppy ears.
•
If a dog breeder wants to produce a true-breeding
strain of floppy-eared dogs that bark on the trail,
•
how should she proceed, knowing that the alleles for
erect ears and silent trailing are present in her
•
kennels?
• P cross = Homozygous Dominant X
Homozygous Recessive
P=
F1=
F2=
Other Types of
Inheritance
• Not all genes are dominant or recessive
• Sometimes get both alleles, a modified
form or the dominant or something new
altogether
Incomplete Dominance
• Two alleles produce three phenotypes
• Third phenotype is a muted version of the
“dominant”
• Example 4 O’clock flowers
RR = Red
RW= Pink
WW = White
Codominance
• When two alleles are both equally
expressed in the heterozygote
• Example : ABO bloodtypes
IAIA, IAi =A type blood
IBIB, IBi = B type blood
IAIB = AB type blood
ii = O type blood
Sex Determination
• In all animals : Sex is determined by the
combination of sex chromosomes (23rd
pair in humans)
• XX = female
• Xy = male
Polygenic Inheritance
• Traits are governed by more than one
gene
• Most traits are polygenic
• Each gene may have two or more alleles
• Many genes can be codominant or
incompletely dominant
• Heterozygotes are intermediate in
phenotype
• A recessive gene combination will effect
the phenotype
Multiple Alleles
The Fruit Fly
Drosophila melanogaster
Homo sapiens