quiz #2 Flashcards
Chromosome
Each chromosome in a pair codes for the same genetic trait but they provide different details
Genes
A segment or band on a chromosome that determines a particular characteristic. Each gene has different possible forms called alleles.
Alleles
Gene: eye colour
Possible alleles: brown or not brown
(Different versions of the gene)
-since we have different chromosomes, we have pairs of alleles that determine a particular characteristic
-one of the two alleles is dominant over the other
The dominant allele
Represented by a capital letter (B) and is always represented in the physical characteristics of the individual (if present)
The recessive allele
Represented by a lower case letter (b) and is only represented in the physical characteristics of the individual if the dominant allele is present
Dominant alleles
- brown eyes (B)
- dark hair (D)
Recessive alleles
- not brown (blue or green) eyes (b)
- blonde hair (d)
The three possible combinations for each pair of alleles…
- BB
- Bb
- bb
Homozygous (pure bread)
- an individual in which both alleles are the same
- homozygous dominant: BB
- homozygous recessive: bb
Heterozygous (hybrid)
- an individual in which both alleles are different
- heterozygous:Bb
Genotype
The genetic makeup of the individual (BB, Bb, or bb)
Phenotype
The physical appearance of the individual (brown eyes or blue eyes)
-the homozygous dominant (BB) and heterozygous genotypes (Bb) will have the same phenotype because the dominant allele is present
History of genetics: who is Gregor Mendel?
- father of modern genetics
- his work was published in 1866, but did not become popular until the 1900’s
- 1822-1884
- he was an Austrian monk who used garden pea plants to explain inheritance of characteristics
- he examined seven different pea traits
- each trait had only two possible variations (alleles)
- Mendel was the first individual to propose the principle of dominance, the law of segregation and the law of independent assortment
Mendel’s three major discoveries:
- characteristics/traits are governed by paired but individual factors (genes)
- these “factors” may be dominant or recessive
- the factors combine to produce characteristic ratios in the later generations
Principle of Dominance
When individuals with different alleles reproduce, the offspring will only express the dominant trait
Law of Segregation
Each gene (allele) separates from the other so that the offspring get only one gene from each parent for a given trait
Mendel’s gentil model plants: he worked with peas because:
- they are easy to grow
- crosses (mated) true breeding peas; pea plants that inherited the same traits generation after generation
- have many simple traits that distinguish strains of pea plants from each other
- this includes height (short or tall), seed colour (yellow or green) and seed shape (wrinkled or round)
Test crosses
- mendel’s experiments with pea plants required him to pollinate flowers
- this refers to as; crossing (fertilizing the flowers reproductive organ)
Generation terms: P
- P generation
- parental
F1
- F1 génération
- first filial
- the children of P
F2
- F2 generation
- second filial
- the grandchildren of the P generation
Punnet Squares
- chart used by geneticists to determine the probability of results when two individuals are crossed
- possible gametes produced by the parents are written at the top and side
- the gametes are combined to produce all of the possible F1 genotypes
F2 generation
To find the F2 generation, you cross the the offspring produced in the F1 generation
-F2 will always have a genotypic ratio of 1:2:1 (1 homozygous dominant…) and a phenotypic ratio of 3:1 (3 dominant, 1 recessive)
Dihybrid cross
Involves 2 genes each consisting of heterozygous alleles
9:3:3:1
Phenotypic ratio when both parents are heterozygous
Inheritance patterns
If genes are located on separate chromosomes they will be inherited independently of one another
-traits are passed down independently
Solving Dihybrid Problems
- Parents’ genotypes
- Possible allele combinations (possible gametes)
- Punnet square cross
- Ratios
Complete dominance
A situation where an allele will determine the phenotype l, regardless of the presence of another allele
Incomplete dominance
- in between
- blending
- neither wants to be dominant
- a situation where neither allele dominates the other and both have an influence on the individual; results in partial expression of both traits
Codominance
- cooperating
- both want to be dominant
- a situation where both alleles are expressed fully to produce offspring with a third phenotype
3 blood type alleles
- I^A
- I^B
- i
The 4 blood types and their genotypes
- A: I^A I^A or I^A i
- B: I^B I^B or I^B i
AB: I^A I^B
O: ii
Which blood types demonstrate dominant and codominant traits
-human blood
Pedigrees
- diagrams that illustrate the genetic relationship among a group of related individuals
- the genotypes are determined by the phenotype
- using pedigrees and Mendelian genetics, we can determine whether a particular trait is caused by dominant, recessive, autosomal or sex linked allele
- experimental crosses are not possible in humans
- medical, historical and family records can be evaluated to study crosses that have already occurred
- records from several generations can be assembled to form family pedigrees
- when writing genotypes put both options
Pedigree symbols
- mating line
- offspring line
- sibling line
- Roman numerals represent generations
- numbers represent individuals within a generation
- the sibling line is in order from oldest (left) to youngest (right)
- solid symbols indicate that they exhibit the trait of interest- it does not always mean that the trait is dominant
Dominant traits in pedigrees
- will be present in each generation
- an individual that represents a dominant trait will have at least one parent that exhibits that dominant trait
Recessive traits in pedigrees
- aren’t always present in each generation
- an individual that exhibits a recessive trait does not have to have a parent exhibit that trait (both parents will be heterozygous carriers of that trait)
Chromosomes
- different organisms have different numbers of chromosomes
- humans have 1 pair of sex chromosomes (X or Y) and 22 pairs of autosomes (non sex chromosomes)
Autosomal inheritance
- when an allele for a trait is found on an autosome
- males and females affected equally since there’s no difference between autosomes of males and females
Sex linked inheritance
Alleles causing genetic disorders found on X and Y chromosomes (sex chromosomes)
- females (XX)
- males (XY)
X and Y Chromosomes
- sections of the X and Y chromosomes are homologous
- however few genes (alleles) are common to both
X linked disorders
- phenotypic expression of an allele that is found only on the X chromosome
- a recessive allele for a disorder located on the X chromosome is more likely to express itself in males than females
A male inherits X chromosome from his mother who carries the recessive allele:
He will express the disorder because the Y chromosome cannot mask the effects of that allele
Males cannot inherit an X linked disorder from father because:
Fathers only pass on Y chromosomes to sons
Females must inherit two copies of the recessive allele (one on each of the X chromosomes):
In order to express the disorder
Y-linked disorders
- passed from father to son only
- fewer Y linked disorders than X linked because Y chromosome is small and does not carry as much genetic info as X chromosome
- ex: hemophilia, colour blindness, duchenne muscular dystrophy
Representing sex linked chromosomes
- males (XY)
- females (XX)
- pick a letter to represent the gene
- establish what is dominant and what is recessive
