genetics & inheritance Flashcards
what is a gene
a gene is a section of DNA, that codes for a specific amino acid
what is an allele
a different version of the same gene
what is a genotype
the genetic constitution of an organism
what is a phenotype
the expression of this genetic constitution and its interaction with the environment
what can alleles be
recessive, dominant or codominant
what can the alleles be in a diploid organism
may be either homozygous or heterozygous
define homozygous `
if the two copies of a gene are the same allele
define heterozygous
if the two copies of a gene are different alleles
give an example of each
homozygous dominant
heterozygous
homozygous recessive
BB
Bb
bb
define codominant alleles
equally expressed within the phenotype
so they would start off with two equal pairs (homozygous) which would end up creating all heterozygous so in that sense they are equally expressed within the F1 generation
what is the ratio for codominance
1:2:1
what is the ratio for monohybrid
3:1
define monohybrid
single characteristic
what are multiple alleles
more than 2 alleles for the same gene, this produces hierarchy of dominance
what is monohybrid inheritance
the inheritance of a single gene, which determines a single characteristic
how are gametes indicated
by drawing a circle around the allele
the fusion of gametes is…
random
what is a gene pool
all the alleles within an interbreeding population (at a specific time)
what are the 2 different types of hardy-weinberg equations
what do the letters represent
p + q - 1
p = dominant
q = recessive
p2 + 2pq + q2 = 1
p2 = homozygous dominant
2pq = heterozygous
q2 = homozygous recessive
What does Hardy-Weinberg principle predicts that
The allelic frequencies (of a particular gene ) from one generation to the next will remain constant
if there is NO :
migration i.e, the gene pool is isolated and there is no flow of other alleles in or out
gene mutations
selection for or against a particular allele
in addition :
there should be a large population
mating within the population should be random (all individuals have an equal chance of mating)
important - highly unlikely that any natural population meet all these requirements
state three causes of genetic variation
mutation
crossing over
independent segregation
random fusion of gametes
what is meant by a genome
all the DNA in a cell
all the genes/alleles in a cell
the total number of DNA bases in a cell
how do multiple alleles of a gene arise
mutations
which are different/at different positions in the gene
what is meant by a recessive allele
only expressed in the homozygote
in genetic crosses, the observed phenotypic ratios obtained phenotypic ratios obtained in the offspring area often not the same as the expected ratios
suggest two reasons why
small sample size
fusion of gametes is random
linked genes; sex-lineage/ crossing over
epistatis
lethal genotypes
define gene lineage
(genes/loci) on same chromosome
define epistasis
the allele of one gene affects or masks the expression of another in the phenotype
described why observed phenotypes don’t match expected values
fertilisation is random
OR
fusion of gametes is random;
small population/sample;
selection advantage/disadvantage/lethal alleles;
rules for dominant alleles
affected offspring MUST have at least one affected parent.
unaffected parents ONLY have unaffected offspring.
if both parents are affected and have an unaffected offspring, both parents must be heterozygous
rules for recessive alleles
unaffected parents can have an affected offspring (if they are heterozygous)
male offspring are more likely than females to show recessive sex-linked characteristics. explain why
(recessive) allele is always expressed in males
females need two recessive alleles
expected offspring phenotype ratios from heterozygous parents:
monohybrid
dihybrid
epistasis
autosomal linkage
dominant : recessive
3:1
9:3:3:1
9:4:3 or 15:1 or 9:7
3:1 (no x over) (no other pattern other than 4 phenotypes with recombination of alleles)
State three causes of genetic variation
• Mutation
• Crossing over
• Independent segregation / assortment (of homologous chromosomes)
• Random fusion of gametes / fertilisation / mating
What is meant by a genome?
• (All) the DNA in a cell/organism;
• ‘(all) the ‘genes’/alleles’ ‘genetic material/code’ in a cell/organism/ person’
• ‘the total number of DNA bases in a cell/organism’
What is a gene pool?
• All the alleles in a population;
How do multiple alleles of a gene arise?
• mutations;
• which are different / at different positions in the gene;
In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios.
Suggest two reasons why.
• Small sample size;
• Fusion/fertilisation of gametes is random;
• Linked Genes; Sex-linkage / crossing over;
• Epistasis;
• Lethal genotypes;
What is meant by a recessive allele?
• Only expressed in the homozygote / not expressed in the heterozygote / not expressed if dominant present;1
What does Hardy Weinberg’s equation predict
• The frequency/proportion of alleles (of a particular gene);
• Will stay constant from one generation to the next/over generations / no genetic change over time;
• Providing no mutation/no selection/population large/population genetically isolated/mating at random/no migration;
Define gene linkage
• (Genes/loci) on same chromosome;
Define epistasis
• The allele of one gene affects or masks the expression of another in the phenotype;
Describe why observed phenotypes don’t match expected values
• Fertilisation is random
• OR
• Fusion of gametes is random;
• Small/not-large population/sample;
• Selection advantage/disadvantage/lethal alleles;
Define codominance
• Both alleles expressed in the phenotype;
Rules for Dominant alleles
• Affected offspring MUST have at least one affected parent.
• Unaffected parents ONLY have unaffected offspring.
• If both parents are affected and have an unaffected offspring, both parents must be Heterozygous
Rules for recessive alleles
• Unaffected parents can have an affected offspring (if they are Heterozygous)
Male offspring are more likely than females to show recessive sex-linked characteristics. Explain why.
• (Recessive) allele is always expressed in males / males have one (recessive) allele;
• Females need two recessive alleles / females need to be homozygous recessive / females could have dominant and recessive alleles / be heterozygous;
Expected offspring phenotype ratios from heterozygous parents:
1. Monohybrid
2. Dihybrid
3. Epistasis
4. Autosomal linkage
- 3:1
- 9:3:3:1
- 9:4:3 or 15:1 or 9:7
- 3:1 (no x over) (no other pattern other than 4 phenotypes with recombination of alleles)
What is meant by the term phenotype
• (Expression / appearance / characteristic due to) genetic constitution / genotype / allele(s);
• (Expression / appearance / characteristic due to) interaction with environment;
Explain how a single base substitution causes a change in the structure of a polypeptide
• Change in (sequence of) amino acid(s)/primary structure;
• Change in hydrogen/ionic/disulfide bonds;
• Alters tertiary/30 structure;
