Download Medelian Genetics Notes

WHAT IS GENETICS?
GENETICS is the study of how
traits are passed from parent to
offspring in the form of Genes.
HISTORY!
Gregor Mendel
 Born 1822
 Austrian Monk
 Examined reproduction of pea
plants
 Plants reproductive organs
are called FLOWERS
 A flower has both male and female
parts.
 The pea plants Mendel was working
with were typically TRUE-BREEDING,
meaning they self-pollinated
 EX, TALL pea plants would always
be pollinated by tall pea plants and
produce tall offspring!
WHAT WE KNOW (MENDEL DIDN’T)
Genes – control a heritable feature (characteristic);
Example: Hair color, seed shape, height;
Allele – controls the variation of a feature (trait).
Example: brown, blonde, black hair
REVIEW TIME: What are homologous
chromosomes???
Homologous chromosomes may……
-Both have the same alleles HOMOZYGOUS (aka:
pure or true-breeding)
-Both have different alleles  HETEROZYGOUS
(aka: hybrid)
Mendel’s Idea
 Cross two pea plants with different contrasting traits!
 Ex:
 First cross : Crossed true breeding purple with
true-breeding white plants.
 Called offspring F1 Generation
 Results were that offspring were_100%
PURPLE_
 Had the white allele disappeared????
Mendel’s Law of Dominance
 some alleles over power others. So even if both
alleles are present, we only “see” the dominant
one.
 the “hidden” allele is called recessive
 This only applies to SOME genes, not all
Second cross  two of the purple F1 Offspring
Called offspring the F2 Generation
Results
- 75 % purple
- 25 % were white
 White trait had reappeared!
 “The Traits (genes) Mendel looked at
Mendel’s Law of Segregation
 during meiosis, the pair of alleles in a parent
will separate.
 Only ONE allele for EACH TRAIT will pass from
each parent to the offspring
 Ex. sugar beet preference.
 dominant allele (A) prefers sugar beets
 recessive allele (a) does not.
 Heterozygote produces gametes
 50% chance
 Get A
 Get a
Question: If a heterozygous sugar beet eater
marries a non-sugar beet eater, what
possible offspring could they have?
Mendel’s Law of Independent Assortment
 Alleles for different genes are passed to offspring
independently of each other.
 The result is that new combinations of genes present
in neither parent is possible.
 How many allele combinations could the following
genotype produce?
 RRYY
 RRYy
 RrYy
Genetic Terms
 Diploid (2n)- Two sets of chromosomes.
 Somatic Cells
 Haploid (n)- One set of homologous Chromosomes
 (gametes)
 Egg- Female haploid gamete
 Sperm- male haploid gamete
Parent – Seriously, you should know this
Meiosis – Cell division that produces haploid gametes
Testes – Site of male meiosis
Gamete – Haploid sex cell (sperm, egg, pollen)
Zygote- Single cell (result of sperm and egg)
Progeny - Offspring
Offspring – see above
Fertilization – gametes fuse into zygote
Ovary- site of female meiosis - eggs
 Genotype: the alleles that an organism has.
- alleles are abbreviated using the first letter of the
dominant trait.
- capital letter represents the dominant 
- ex:
P for purple flower allele
- lower case represents the recessive. 
- ex:
p for white flower allele
 All diploid organisms have two alleles for each trait:
 Can be two of the same alleles  Ex: PP or pp
called Pure or Homozygous.
OR
 Can be two different alleles 
 Ex: Pp described as Hybrid or Heterozygous
Phenotype: physical appearance
 Examples: brown hair, widows peak, purple
flowers
 the trait that “wins” in the case of complete
dominance;
 depends on the combination of alleles
GENOTYPE
MENDEL’S CROSSES
 P Generation: “parents;”
 F1 Generation  offspring of P generation
 F2 Generation  offspring of F1 generation
Punnet Squares 
How we show allele combinations in crosses
Allele in sperm 1
Allele in sperm 2
Allele in Egg 1
Allele in Egg 2
Zygote formed if
sperm 1 fertilizes
egg 1
Zygote formed if
sperm 1 fertilizes
egg 2
Zygote formed if
sperm 2 fertilizes
egg 1
Zygote formed if
sperm 2 fertilizes
egg 2
Monohybrid Cross
Tall vs. Short Example
 Tall allele  T Short allele  t
 P Cross
TT
x
tt
T
 F1 Generation
 Genotypes
 Phenotypes
t
t
T
F2 Generation
F1
Generations 100% Tt
Tt x
Tt
T
F2 Generation
GenotypesRatio =
PhenotypesRatio =
T
t
t
Sample Problems
 Homozygous Tall x Heterozygous Tall
 Heterozygous Tall x Homozygous Short
Probability
 Probability is only the LIKELIHOOD of an event
happening.
 It does not mean it is what HAS to happen
 Ex. Coin Toss. Two tosses, always one heads and
one tails?
 What happens when we look at very large
samples?
 Ex. Male/female ratio of a family vs. the world!
INHERITENCE PATTERNS
 Every gene demonstrates a distinct phenotype when
both alleles are combined (the heterozygote)
 Complete dominance is when both alleles are present,
only the dominant trait is seen.
 Incomplete dominance - when both alleles are present,
the two traits blend together and create an
intermediate trait
INCOMPLETE DOMINANCE
Inheritance Patterns:
Co-dominance
- when both alleles are present, both traits are visible
Different notation: Use first letter of the feature with a
superscript for the trait.
Example: CW or CR for white petals or red petals;
Women have two
X’s but men only
have one.
How do we deal with
the genes on the X
chromosome?
Probabilities
 Question 1: What is the probability of having a
female offspring?
 Question 2: After having 4 sons in a row, what is
the probability the next kid will be male?
 Question 3: What is the probability of having
three daughters in a row?
Sex-Linked Traits
 Refers to traits coded by genes found on the X
chromosome
 Females will have 2 copies of these genes
 Males will have 1 copy of these genes
 Significance???
 If males get a bad (recessive) allele for a sexlinked trait, THEY WILL EXPRESS THE
RECESSIVE TRAIT!
Example – Color Blindness
 Seeing color (XC) is dominant to being color blind
(Xc)
 Identify the sex and trait of the following:
 XCY
 XcXc
XCXc
XCXC
XcY
Cross Number 1:
XC
XC
Y
Xc
C
C
X X
C
c
X X
XC Y
c
X Y
What % chance
of having color
blind daughter?
Son?
SEX-LINKED TRAITS
COLOR BLINDNESS
AFFLICTS 8% MALES AND 0.04% FEMALES.
Test cross: a cross that determines genotype of
dominant parent
- Cross unknown dominant parent (possibilities BB
or BB) with a recessive parent then analyze the
offspring.
Ex. B- Black Hair
b- white hair
You are given a black-haired guinea pig and need to
determine whether homozygous dominant or
heterozygous.
Multiple Alleles
 Genes may have more than two alleles.
Multiple alleles: Some genes have more than two
variations that exist, although we still only inherit 2
Example: Human blood types
Three alleles:
IA
IB
i
Genotype
IAIA
IAi
IBIB
IBi
IAIB
ii
Phenotype
A
A
B
B
AB
0
Polygenic –
Multiple genes code for a
trait each with 2 alleles
Examples in humans:
Skin Color
Eye Color
Height
Why so many possibilities???
SKIN PIGMENTATION
Dihybrid cross:
A cross that focuses on possibilities of
inheriting two traits
- two genes, 4 alleles
Black fur is dominant to brown fur
Short fur is dominant to long fur
What is the genotype of a guinea pig
that is heterozygous for both black and
short fur?
Dihybrid cross:
Parent phenotypes: BbSs x BbSs
Figure out the possible gametes:
Then set up punnett square
Dihybrid cross:
BS
BS
Bs
bS
bs
Bs
bS
bs
Di-hybrid Cross Generalization
 Laws of probability indicate a 9:3:3:1 phenotypic
ratio of F2 offspring resulting in the following:
 9/16 of the offspring are dominant for both traits
 3/16 of the offspring are dominant for one trait
and recessive for the other trait
 3/16 of the offspring are dominant and recessive
opposite of the previous proportions; and
 1/16 of the offspring are recessive for both traits.
Linkage and Gene Maps
 When genes are one separate chromosomes, they
independently assort.
 If on the same chromosome, they will rarely
separate and be inherited together (gene
linkage)
 Actually it is the chromosomes that assort
independently, not the genes. Mendel was just
lucky with the genes he was looking at!
 Crossing over in meiosis often separates linked
genes.
 The distance between the two genes on the same
chromosome are from each other affects the
frequency of separation from each other during
crossing-over.
 Further Apart
 Closer together 
 The frequency of crossing over between genes is actually an
indicator of how far apart different genes are located from each
other on the same chromosome.
 Use the frequency rates to make gene maps that show relative
locations of genes with respect to each other.