Unit 5 - Heredity Flashcards
What are the P, F1, and F2 generations?
P = parental generation; true-breeding parents in a genetic cross
F1 = first filial generation; offspring of P parents
F2 = second filial generation; offspring of F1 parents
What is the law of independent assortment? Which phase of meiosis is it exhibited in?
DESCRIPTION
Each pair of alleles for different genes will sort independently during gamete formation
In other words, the allele a gamete receives for one gene (blue eyes) does not influence the allele received for another gene (sharp nose).
MEIOSIS PHASE
It is exhibited in metaphase I of meiosis because the maternal and paternal chromosomes of each homologous pair line up in random order at the metaphase plate
Alleles
Definition: Alternative versions of genes with slightly different DNA sequences
Alleles account for variations in inherited characteristics among offspring
EX: Gene/characteristic = flower color and alleles = white and purple
Dominant vs Recessive allele (normal case)
If the two alleles are different, the dominant allele will be fully expressed in the offspring and determine its phenotype while the recessive allele will have no noticeable effect
What is the law of segregation? Which phase of meiosis is it exhibited in?
DESCRIPTION
The two alleles for a character segregate (separate) during gamete production and end up in different gametes.
An egg/sperm gets only the maternal allele or only the paternal allele which were both present in the somatic cell
If the parent has two of the same alleles from the mother and father (true-breeding), then the offspring will all get that version of the gene, but if the parent has two different alleles, each offspring has a 50% chance of getting one of the two alleles
MEIOSIS PHASE
It’s exhibited in anaphase I of meiosis because the separation of homologs means the separation of alleles
Phenotype
An organism’s expressed physical traits (EX: purple color)
Genotype
An organism’s genetic makeup (EX: PP)
What are the rules of probability?
To find the chances of event 1 AND event 2 = MULTIPLY
To find the chances of event 1 OR event 2 = ADD
What is complete dominance?
Heterozygous and homozygous dominant organisms are indistinguishable. Could occur when just one single dominant allele produces a sufficient amount of material/protein/substance to support the individual
What is incomplete dominance? Give an example.
Neither allele is completely dominant, so F1 hybrids will have a phenotype somewhere between those of the two parental varieties
EX: red flowers + white flowers –> pink flowers (R:P:W = 1:2:1)
What is codominance? Give two examples.
Two alleles are dominant and both are expressed on the phenotype of the offspring
EX: brown cow + white cow –> brown and white spotted cow
EX 2: Human blood types - alleles A and B are codominant; someone who has both alleles has blood type AB
What are multiple alleles? Give an example.
Genes can exist in more than 2 allelic forms
EX: There are 3 possible alleles for human blood types (IA, IB, and i) but one person can receive any combination of 2 alleles
What is pleiotropy? Give an example.
Property of a gene that causes it to affect more than one phenotypic character (opposite of polygenic inheritance)
EX: Sickle cell disease has multiple symptoms all caused by a single defective gene
What is polygenic inheritance? Give an example.
Multiple genes have an additive effect on a single phenotype character (opposite of pleiotropy)
the character varies in a spectrum and does not have discrete varieties
EX: skin color, height
What is the difference between dominantly inherited disorders and recessively inherited disorders?
Dominantly inherited disorders require only one copy of the allele (defective gene) in order for the disorder to be expressed while recessively inherited disorders require two copies
Chromosome theory of inheritance
Mendelian genes have specific loci (locations) along chromosomes, and it is the chromosomes that undergo segregation and independent assortment
Where are sex-linked genes found?
They are located on a sex chromosome (X or Y in humans)
Wildtype
The phenotype for a character most commonly observed in natural populations (EX: red eyes in drosophila)
Alternative traits are called mutant phenotypes
Why does X inactivation occur and how does it work?
PURPOSE
Allows males and females to both have one active copy of X-linked genes, so they both produce the same amount of proteins
PROCESS
Females inherit two X chromosomes but one of them is randomly chosen in each cell of the body to become inactivated during embryonic development. This occurs through methylation.
The inactive chromosome condenses into a Barr body, which lies along the inside of the nuclear envelope. (# of barr bodies = total chromosomes for a trait - 1)
However, they are reactivated in the cells that give rise to eggs so that every female gamete will have an active X chromosome after meiosis.
All mitotic descendants of the cell have the same inactive X chromosome
Where are linked genes found? What unique pattern of inheritance do they exhibit?
Linked genes are located on the same chromosome and therefore tend to be inherited together in genetic crosses
EX: red hair and freckles
Definition of genetic recombination
Production of offspring with a new combination of genes not seen in the P generation (nonparental phenotypes)
This is due to crossing over during prophase I of meiosis
What are the two types of offspring phenotypes created by genetic recombination?
Parental phenotypes: offspring with phenotypes the same as one of the parents
Recombinants: offspring with phenotypes different from either parent
How does recombination frequency relate to linked genes?
50% or more recombinant frequency = more crossing over = genes are located on different chromosomes and are inherited independently
less than 50% recombinant frequency = less crossing over = genes are located on the same chromosome and are linked
Why do linked genes exhibit limited crossing over?
Since linked genes are closer together, there are fewer points for crossing over to occur
What is the formula for recombinant frequency?
recombinant frequency = (# of recombinants / total offspring) x 100%
How does the distance between genes relate to recombinant frequency?
The farther apart two genes are, the higher the probability that crossing over will occur between them, and thus the higher the recombinant frequency
Map unit
Used to express relative distances between genes along a chromosome on a linkage map
1 map unit = 1% recombinant frequency
List the four types of chromosomal mutations.
- Deletion
- Duplication
- Inversion
- Translocation
Deletion
Occurs when a chromosomal fragment is lost, resulting in a chromosome with missing genes
Duplication
Occurs when a chromosomal segment is repeated, which can alter gene dosage and be harmful
Inversion
Occurs when a chromosomal fragment breaks off and reattaches to its original position in reverse orientation
ABC –> CBA
Translocation
Occurs when chromosomal fragments of nonhomologous chromosomes swap
What is the difference between chromosomal mutations and nucleotide mutations?
Chromosomal mutations are much more large-scale because they involve entire regions of DNA and numerous genes. Nucleotide mutations oftentimes only deal with one base.
How do you determine the inheritance pattern of a trait by looking at a pedigree?
- Sex-linked or Autosomal
Observe if the trait is inherited by both males and females or only one sex. If there is no distinct preference, it’s autosomal
- Dominant or Recessive
If both parents have a recessive phenotype, all of their offspring should also have a recessive phenotype. If both parents do not have the trait (heterozygous carrier) and the child does, it is recessive.
If the trait is dominant, one of the parents of the affected organism must have had the trait.
What is the phenotypic ratio of a dihybrid cross between individuals heterozygous for both traits? (assume complete dominance)
9:3:3:1
What is the phenotypic ratio of a monohybrid cross between heterozygotes? (assume complete dominance)
3:1
If there is sex-linked recessive inheritance and there is an affected male, which parent donated the affected allele?
Mother
For sons, the father donates the Y chromosome while the mother donates the X chromosome
What is the phenotypic ratio of a dihybrid cross between one heterozygote and one homozygous recessive individual? (assume complete dominance)
1: 1:1:1
* if this is not the case, the genes are LINKED
Explain how it is possible to have a chromosome that includes BOTH your grandparents’ DNA.
Your mother inherits one member of each homologous pair from your grandparents. During gamete formation in your mother, crossing over occurs between nonsister chromatids in each homologous pair. Thus, each chromosome formed and passed on to you from your mother contains DNA from both your grandparents.
