Patterns of inheritance and variation Flashcards
Define gene
A sequence of DNA bases that code for an amino acid.
Define alleles
Different versions of the same gene.
Define genotype
An organism’s genetic makeup
Define phenotype
An organism’s physical characteristics determined by its genotype and the environment
Define dominant
An allele which is always expressed in the phenotype
Define recessive
An allele which is only expressed when it is homozygous in the genotype
Define locus (plural: loci)
The specific position of a gene on a chromosome
Define homozygous
An organism with two identical alleles for a trait
Define heterozygous
An organism with two different alleles for a trait
Define codominant
Both alleles are equally dominant and expressed in the phenotype
Define multiple alleles
More than two alleles for a single gene
Define monohybrid/monogenic
Inheritance involves the transmission of one gene from parents to offspring
Define dihybrid
Inheritance involves transmission of two genes from parents to offspring
Define sex linkage
A gene whose locus is on the X chromosome
Define autosomal linkage
Genes which are located on the same chromosome (but not the sex chromosomes)
Define epistasis
When on gene modifies/masks the expression of a different gene at a different locus.
What is the F1 generation in monohybrid crosses?
- Offspring from a cross between individuals with homozygous dominant and homozygous recessive alleles.
- All offspring are heterozygous.
- All offspring express dominant allele in their phenotype
Describe the F2 generation in monohybrid crosses
- Offspring from a cross between two F1/heterozygous individuals.
- Results in offspring displaying 3:1 ratio of dominant to recessive phenotypes.
How to show codominance in genetic diagrams.
- Uppercase letter is used to signify the gene
- Superscript uppercase letters indicate the alleles.
Importance of multiple alleles
Increases the phenotypic diversity among individuals in a population
Which gene controls the ABO blood group system?
Immunoglobulin/ I gene.
If a genotype is IAIA or IAIO what is the phenotype and which antigens are present?
Blood type A
A
If a genotype is IBIB or IBIO what is the phenotype and which antigens are present?
Blood type B
B
If a genotype is IAIB what is the phenotype and which antigens are present?
Blood type AB
A and B antigens
If a genotype is IOIO what is the phenotype and which antigens are present?
Blood type O
None
How to identify offspring phenotypes with multiple alleles.
- State the parents gametes (e.g. heterozygous for blood group A has gametes IA and IO)
- Use a punnett square to identify potential phenotypes for offspring
-State the phenotypic ratio
Describe the inheritance of sex in humans.
Humans have 23 pairs of chromosomes, 22 of these are autosomes so are identical in males and females. The 23rd pair are sex chromosomes which differ between the sexes and determine an individuals sex.
What two forms can the sex chromosome come in?
Name and describe
X chromosome - found in both males and females.
Y chromosome - found in males only.
- Female body cells have two X chromosomes, XX so all egg cells contain an X chromosome.
- Male body cells have an X and a Y chromosome, XY, so all sperm cells have an X or a Y chromosome.
What are most sex linked genes? Why? What does this mean?
- X linked
- X chromosome is larger in size
- Recessive alleles tend to appear more in the phenotype of males as there is no corresponding allele on the Y chromosome to mask them.
What is haemophilia?
An X-linked recessive disorder caused by a defective gene on the X chromosome.
- alters DNA sequence for crucial blood clotting protein resulting in excessive bleeding.
Facts about the inheritance of haemophilia
- Mostly affects males/ doesn’t often affect females.
- Always inherited from the mother in males
- Mostly inherited from carrier mothers
- Affected Fathers can only pass it on to daughters.
How do linked genes (located on the same autosome) tend to be inherited in offspring?
Tend to be inherited together in offspring rather than assorting independently.
Autosomal linkage means that…
- There is a non-random association of alleles at different loci
- Parental allele combinations are preserved across generations
- Phenotypic ratios observed in dihybrid crosses are different from those expected in independent assortment.
The effects of crossing over on autosomal linkage
- Crossing over may separate linked genes
- When genes ARE linked, fewer recombinant offspring tend to be produces, resulting in less genetic variation being introduced by crossing over.
- The closer linked genes are on a chromosome, the more likely they are to be inherited together.
What is the recombinant frequency?
A measure of how often recombinant offspring are produced as a result of crossing over.
Recombination frequency formula
Recombination frequency = number of recombinant offspring/ total number of offspring
What does a 50% recombination frequency indicate?
That there is no linkage between genes
What does a recombination frequency below 50% indicate?
Signifies some degree of autosomal linkage
What does a lower recombination frequency indicate?
The lower the recombination frequency, the closer the genes are on a chromosome meaning they are less likely to be separated in crossing over.
