Testcrosses Flashcards
What is a testcross?
an individual with a dominant phenotype and an unknown genotype is bred with a homozygous recessive individual and then look at the offspring to determine the unknown genotype
What is incomplete dominance?
neither gene is dominant over the other so the 2 traits are blended together
Give an example of incomplete dominance.
When a red snapdragon is crossed with a white snapdragon, neither gene is dominant, so there is a blending of the two colors. A pink snapdragon is created.
How do you represent incomplete dominance in characters?
C superscript-r C superscript-w. The superscripts vary with the traits.
When a homozygous red radish plant is crossed with a homozygous white radish plant, purple radishes result. What kind of crossing is this? And what is the phenotypic and genotypic ratio of a cross between 2 plants from the F1 generation of the cross described above?
a) incomplete dominance
b) the F1 generation is the result of the 1st cross. These plants would be purple: CrCw. Crossing to of these plants together would develop a Punnett square like this:
Cr Cw
Cr CrCr CrCw
Cw CrCw CwCw
The phenotypic ratio of this cross is:
1 red: 2 purple: 1 white
The genotypic ratio of this cross is:
1 CrCr: 2 CrCw: 1 CwCw
What is codominance?
Both traits are expressed
Example of codominance.
roan cow. White fur and red fur combined to create a blotchy cow (a roan cow)
Compare and contrast codominance and incomplete dominance.
- described in same characters, cross them in the same way
2. incomplete is a blending, co = both traits are expressed
What is a hybrid? a monohyrbid cross? a dihybrid cross?
hybrid- combination of genes, creating F1 generation
monohybrid- looking at one gene/trait, simple Punnett square
dihybrid- comparing two traits
Give an example of a dihybrid cross.
yellow round peas and green wrinkled peas
YYRR- yellow round
yyRR- green round
Punnet square with 16 boxes
What are multiple alleles?
more than 2 possible alleles for a particular trait
Give an example of them.
Blood type- I(A) I(B) i codominance- I(A)I(B) different variations- I(A)I(A) or I(A)i for type A blood, same for B, look above for AB, ii for O blood
What is pleiotropy?
multiple phenotypic effects of a gene
Give an example of pleiotropy.
sickle cell anemia - red blood cells are shaped like a crescent moon because they don’t carry O2 properly. This causes them to clop in arteries. One change in the DNA causes one amino acid to misshape which causes hemoglobin (the protein that carries O2) to misshape which results in misshapen red blood cells
What is epistasis?
a gene at one location affects the phenotypic expression of another gene at a second location
Give an example of epistasis.
labs- yellow, brown, black
black- when B and E gene are dominant
brown- when b gene is recessive and E gene is dominant
yellow- when E gene is recessive, it blocks the B gene’s expression so it does not matter if the B gene is dominant or recessive
What is the purpose of the A series of genes in a bunny? Describe the genes in the A series.
determines how many colors there will be on a shaft of fur
- A- agouti
- a(t)- tan pattern
- a- self
in order from dominance to recessive-ness
can have any variation of those genes
i.e. Aa(t) = agouti, AA = agouti, Aa = agouti
B series (purpose and description)
basic color, B (black), b (brown)
C Series
C- full color development (7 pigments: 4 dark/ 3 yellow)
c(chd) - dark chinchilla (5 pigments: 4 dark/ 1 yellow)
c(chl)- light chinchilla (2 pigments: 2 dark/ 0 yellow)
c(h)- himalayan (dark extremeties - tail, nose, ears)
c- albino (color expression blocked completely)
D series
D- dense color
d- dilute color
E series
incomplete dominance
E(subscript n) - broken
e (subscript n) - solid
E(n)e(n) is when they’re the most mixed.
What is polygenic inheritance?
multiple genes control a single phenotypic characteristic, opposite of pleiotropy
Example of polygenic inheritance
skin color: AABBCC = more dominant, more dark
aabbcc = more recessive, more light
What are multifactorial characters?
traits that are affected by both genes and the environment
Give 2 examples of multifactorial characters?
hydrangeas- color affected by temperature
himalayan bunnies- hotter = paler dark spots, colder = spots grow darker
What are pedigrees?
family history or tree that explains how genes get passed from one generation to the next
What are carriers?
heterozygous for a recessive allele but have the dominant phenotype, recessive allele can be expressed in next generation
Why do doctors perform amniocenteses and chorionic villus samplings?
doctors can do these on babies to determine if they have a disease
Describe amniocenteses.
amniotic fluid around the baby contains body cells, take some of this out with a needle through the stomach, grow cells, develop karyotype
Describe chorionic villus samplings.
suction tube through the cervix, gather some tissue off the chorionic villi of the placenta, grow cells, develop karyotype
How did Mendel get lucky?
There were no alternate variations of genes (incomplete, codominance). Only 2 possible pairs of results (yellow/green). law of independent assortment: genes were found on different chromosomes so they separated independently from one another
What did the 2 pennies represent?
the parents’ gametes coming together in fertilization to create a zygote
When you draw a Punnett square, what do the letters represent?
the possible gametes for your genotype
What are Mendel’s 4 important findings?
- There are “factors” in living things that are inherited by offspring.
- These “factors” occur in pairs. Today we know these “factors” are genes. The alternate versions of a gene are called alleles.
- Dominant factors are always expressed. Recessive ones have no effect.
- the law of segregation- the 2 factors for a trait separate during gamete formation (anaphase).
