Blending inheritance (genetic material from two parents mixes i.e blue + yellow = green)
Inheritance of acquired characters (traits in the parents are modified during their life and then passed on to offspring)
Gregor Mendel
Austrian monk
Started breeding pea plants ~1857
Why did Mendel use peas?
Available in many varieties
Short generation time
Large numbers of offspring (each pea is an offspring)
Could control mating between plants
Basics of Mendel's experiments
Chose traits (i.e pea color) with only 2 variations (yellow vs. green)
Started with pure lines (true-breeding plants)
Crossed-pollinated two contrasting pure lines to get F1 generation
Crossed F1 plants (self or cross) to get F2 generation (most early work stopped at F1)
Why was blending hypothesis out?
No light purple flowers in F1
However the white trait not lost reappears in F2
Mendel's model of inheritance
Proposed particulate inheritance (discrete, unchanging particles are passed to offspring)
Alternative versions of genes account for variation in characters (alleles- differences due to variation in nucleotide sequence and you can have 1 gene with different alleles)
An organism has dominant and recessive alleles
Dominant allele
Determines the organism's appearance
Doesn't mean most common or most fit
Determines the phenotype (if dominant allele is present then organism will express the associated phenotype)
Recessive allele
Has no noticeable effect (recedes/masked)
Homozygous
If an organism has two identical alleles for a gene (dominant PP or recessive pp)
Heterozygous
If an organism has two different alleles for a gene (Pp)
Genotype
Actual genetic makeup (Pp)
Phenotype
Appearance or observable trait (purple vs. white)
Mendel's 1st Law
Law of segregation
Law of Segregation
The two alleles for a gene separate from each other during gamete formation and end up in different gametes
-separation of homologous chromosomes in meiosis
Monohybrid Cross
Both need to be heterzygous and you are only looking at one trait
-produced by crossing true-breeding plants (p)
Phenotypic ration is always 3:1
What was weird with Mendel's results
The data was almost exact with the 3:1 ratio
There is speculation on if he fudge his data bc the odds are less than 1 in 10,000
If he did, he managed to come up with the right numbers without any proof
Dihybrid Cross
Heterozygous for 2 traits
Produced by crossing true-breeding plants that differ in both traits
Imagine crossing 2 pea heterozygous at the loci for flower color (white vs. purple) and seed color (yellow vs. green) with a second pea for homozygous for flower color (white) and seed color (yellow). What types of gametes will the first pea produce?
Four gamete types: white/yellow, white/green, purple/yellow, purple/green
Mendel's 2nd Law
Law of Independent Assortment
Law of Independent Assortment
Each pair of alleles separates independently of every other pair of alleles during gamete formation
i.e the alleles for flower color separate independently of the alleles for plant height
Does the law of independent assortment always hold true?
The law of independent assortment always holds true for genes that are located on different chromosomes, but for genes that are on the same chromosome, it does not always hold true.
Test Cross
Used to determine genotype when an organism expresses a dominant phenotype
You need to cross the unknown (test) individual with a homozygous recessive (rr)
In rabbits, black fur (B) is dominant over white (b). You find a white baby rabbit. What is its genotype?
bb
In rabbits, black fur (B) is dominant over white (b). You find a nest of black baby rabbits. What is the genotype?
Cannot be determined without test cross
In a monohybrid cross (both parents are heterozygous looking at one trait), what is the probability that the first offspring will have purple flowers?
3/4
In a monohybrid cross (both parents are heterozygous looking at one trait), what is the probability that the second offspring will have purple flowers?
3/4
In a monohybrid cross (both parents are heterozygous looking at one trait), what is the probability that the first and second offspring will have purple flowers?
9/16
Multiplication rule
The probability of two (or more) events occurring in a specific combination
And: i.e the 1st toss will be heads and the 2nd toss will be heads
Multiply the probability of each event (1/2 heads * 1/2 heads = 1/4 both heads)
Addition Rule
The probability of two (or more) mutually exclusive events occurring
Or
Add the individual probability
What is the probability of YYRr when crossing YyRr * YyRr
1/4 (YY) * 1/2 (Rr) = 1/8
What is the probability of yyrr when crossing YyRr * YyRr
1/4 (yy) * 1/4 (rr) = 1/16
What is the probability of yellow round when crossing YyRr * YyRr
Y=yellow, y=green and R=round, r=wrinkled
3/4 (YY or Yy) * 3/4 (RR or Rr) = 9/16
What is the probability of YYrrTt when crossing YyRrTt * YYRrtt?
1/2 YY * 1/4 rr * 1/3=2 Tt
1/16
What is the probability of at least two characters being recessive when crossing YyRr * YyRr
YYrrtt: 1/2 yy * 1/4 rr * 1/2 tt = 1/16
Yyrrtt: 1/2 Yy * 1/4 rr * 1/2 tt = 1/16
Total: 2/16 (1/8)
Chromosomal theory of inheritance
Mendel's work went unappreciated and in 1902 scientist came up with an explanation for his results
-Meiosis accounts for Mendel's observations
Mendel's "hereditary determinants" were genes located on chromosomes
Mendelian genes have specific loci along chromosomes (location of a gene on a chromosome)
It is the chromosomes (not individual genes) that undergo segregation and independent assortment
Why did they test on fruit flies?
They produce tons of offspring
Can breed a new generation every two tweeks
Only 4 pairs of chromosomes
Cheap to keep
Early 1900s- Thomas Hunt Morgan began screening thousands of flies for a variant
What was named for the first mutant gene discovered
Fly genes
-Allele for white eyes = w (Xw)
-Allele for wild-type (red eyes) = w+ (XW)
Since the reciprocal cross gave different results, Morgan concluded that eye color and sex must be linked (gene located on sex chromosome)
Sex-linked inheritance
Genes located on a sex chromosome exhibit unique patterns of inheritance
A white-eyed female Drosophila is crossed with a red-eyed male Drosophila. Which statement below correct describes the results?
B) none of the females will have white eyes
In some Drosophila species there are genes on the Y chromosome that do not occur on the X chromosome. Imagine that a mutation of one gene on the Y chromosome reduces the size by half of individuals with the mutation. Which of the following statements is accurate with regard to this situation?
B) This mutation occurs in all male but no female offspring of a male with the mutation
What other chromosomal sex determination systems are in place other than XY?
XO system (Female XX, Male X)
ZW system (Female ZW, Male ZZ)
Haplo-diploid system (Female diploid, Male haploid)
Temperature dependent
What makes a male a male?
X & Y chromosomes segregated into separate gametes (sperm) during meiosis in males
-If X-bearing sperm fertilizes egg = female
-If Y-bearing sperm fertilizes egg = male
Before 2 months, the embryo's gonads are generic.. can develop into ovaries or testes depending on presence of Y chromosome
SRY gene
Sex-determining region of Y
-Identified in 1990
-If the gene is expressed... the gonads develop into testes (codes for a transcription factor)
How did scientists find the SRY gene?
Analyzed sex-reversed individuals
-XX males = has portion of Y that encodes SRY
-XY Females = lack SRY locus of Y chromosome
What on the Y chromosome causes an embryo to be a male?
SRY gene
Why are very few disorders passed from father to son?
Y chromosome contains 78 genes (which only encodes 25 proteins) whereas X chromosomes contains ~1,100 genes
How can an XY be phenotypically female?
Loss of SRY gene
Mutations in other critical genes (i.e androgen receptors)
How can XX be phenotypically male?
Translocation of SRY gene
Why are X-linked recessive disorders far more common in males than females?
Females have 2 copies of the X chromosome so both must contain the defective Allele
Males only have 1 X chromosome so if they inherit a bad copy from their mother they will express it
X-linked Disorders
Color Blindness
Duchenne Muscular Dystrophy
Hemophilia
Linked Genes
Each chromosome (with the exception of Y) carries hundreds or thousands of genes so it is unlikely that genes on the same chromosome would sort independently.
Genetic Recombination
Some offspring have phenotypes that do not match either parent (recombinants)
This is due to crossing over which allows for recombinant of linked genes
The farther apart 2 genes are on a chromosome, the more likely a crossover event will occur
Degrees of Dominance
Characters Mendel chose showed complete dominance
For some genes neither allele is completely dominant
-Incomplete Dominance
-Codominance
Incomplete Dominance
Heterozygotes have intermediate phenotype
This is when the kids have a blending of the two phenotypes of the parents.
Codominance
Neither allele is dominant, both are expressed with no bleeding.
This is shown when animals are spotted
ABO Blood Groups are example of what?
Codominance
Multiple Alleles
Mendel only looked at characters which had two alleles.. most genes have more than 2 alleles
This means more possible combinations of phenotypes
Example: ABO Blood groups
- 3 possible alleles: IA, IB, I
- 4 possible phenotypes: A, B, AB, O
Environmental effects
The phenotype of many traits can be altered/influenced by the environment
Example: pink flowers can change colors because pH in soil effects the color, Siamese cats are all white when born because the dark enzyme only functions in cooler temperatures, height if you are malnourished then you can't grow to potential
PKU-recessive disorder
Lack enzyme to break down phenylalanie
Buildup interferes with development and results in severe mental retardation
Outcome can be prevented if phenylalanine is removed from diet
