Inheritance Flashcards
Gene
length of DNA that codes for a particular polypeptide
Allele
a different version of a gene
there may be many alleles of a single gene (e.g. blood)
Alleles may be dominant, recessive or codominant
Homozygous
when organism has 2 of the same alleles for a gene
Heterozygous
when organism has 2 different alleles for a gene
Phenotype
the expression of the genetic constitution and its
interaction with the environment
Genotype
the genetic constitution of an organism
Genome
all of the DNA in an organism
Recessive
an allele that’s only expressed when 2 copies are present
Dominant
an allele expressed when only when copy is present
codominance
when two alleles both contribute to phenotype
e.g. AB blood type
what is monohybrid inheritance ?
- inheritance of a single gene
what is the basic law of genetics (aka the law of segregation) ?
- in diploid organisms, characteristics are determined by alleles that occur in pairs
- only one of each pair of alleles can be present in a single gamete
what’s a ratio?
a measure of the relative size of 2 groups expressed as a proportion
why are actual results of genetic crosses different from predicted results
discrepancies due to statistical error
what is dihybrid inheritance ?
how 2 characters, determined by two different genes located on different chromosomes, are inherited
what’s a carrier?
- a person carrying an allele which isn’t expressed in the phenotype but can be passed on to offspring
What does diploid mean?
having paired sets of chromosomes in a cell
What does haploid mean?
having a single set of unpaired chromosomes
What kind of organisms are humans?
diploid (we have 2 sets of chromosomes) so we have 2 alleles for each gene
What are gametes?
- sex cells
- contain half the genetic material of the organism
- contain one allele for each gene
What happens when two gametes (from two parents) fuse together?
alleles they contain form genotype of offspring produced
what do monohybrid crosses show?
the likelihood of the different alleles of that gene (so different versions of the characteristic) being inherited by offspring of certain parents
First set of offspring referred to as what?
F1 generation
the second called F2 generation
How to do dihybrid crosses
e.g. seed colour (yellow or green) and seed shape (wrinkled or round) 1 work out parental genotypes 2 write out parental gametes 3 work out offspring genotypes (4 by 4 punnet square.... four possible outcomes: round and yellow round and green wrinkled and yellow wrinkled and green) 4 calculate ratio
why can any four types of gametes (in dihybrid crosses) of one plant combine with any of the four types from another plant ?
because fertilization is random
what is codominance?
both alleles expressed in the phenotype
what are multiple alleles?
where there are more than 2 alleles, of which only 2 may be present at the loci of an individual’s homologous chromsomes)
In a dihybrid F1 generation cross, the phenotypic ratio for the F2 generation is always what ?
9:3:3:1
when does autosomal linkage occur?
if 2 or more genes are located on the same autosome (non-sex chromosome)
- two genes less likely to be separated during crossing over, resulting in the alleles of the linked gene being inherited together
when does sex linkage occur?
- when there is a gene on the X chromosome, not present on the Y chromosome
(meaning males are more likely to exhibit recessive disorders like haemophilia)
What is epistasis?
the interaction between two non-linked genes which causes one gene to mask the expression of other in the phenotype
How do epistatic genes work?
- antagonistically (against each other) or in a complementary fashion
which is the epistatic gene?
- the gene suppressing another gene
what is the hypostatic gene?
- the gene being suppressed
Antagonistic epistasis can be what?
- either recessive or dominant
Describe dominant antagonistic epistatsis
- expression of dominant allele (epistatic gene) prevents expression of hypostatic gene
What is recessive epistasis?
- it occurs when presence of two copies of the recessive allele at the first locus prevents expression of another allele at a second locus
give an example of complementary epistasis?
- two genes work together
for example, they may encode two enzymes that work in succession
sex chromosomes of males and females
females - XX chromosomes
males - XY chromsomes
what makes a gene sex-linked?
- if it is carried on the X or Y chromsome
why are recessive alleles found in females more likely to expressed in their male offsprings?
- X chromosome is much longer than Y, meaning most of the X chromosome doesn’t have an equivalent portion on Y chromosome
- meaning recessive alleles found on this portion of the X chromosome will be
more likely to be expressed
THEREFORE:
recessive phenotypes are more likely to be present in men e.g. haemophilia
