Inheritance Flashcards
adhere to this booklet often to look at the practice questions
define gene
sequence of DNA bases that codes for a specific polypeptide
where are the genes for particular traits found?
found in the same location on the chromosomes (locus) in different individuals of the same species
what are the three main characteristics of a gene?
they can separate and combine
they can mutate
they code for the production of specific polypeptides
what does a diploid individual contain?
contains homologous pairs of chromosomes, one chromosome copy from their mother and one from their father
they therefore have two copies of each gene
define alleles
they are alternative forms of genes occupying a similar locus on homologous chromosomes
what are the three different allele combinations for any one gene?
heterozygous - having different alleles for a given gene (i.e. a dominant and a recessive allele are present together)
homozygous dominant - having two dominant alleles for a given gene
homozygous recessive - having two recessive alleles for a given gene
what is the gene pool?
all the alleles in a population at any one time are called the gene pool
define genotype
the genetic make-up of an individual; all of the alleles that they possess
define phenotype
the observable characteristics of an organism determined by the genotype
in the simplest situations, a particular characteristic is controlled by a single gene with two alleles
therefore, if an organism is homozygous dominant or heterozygous for a particular characteristic; the dominant allele will always be expressed in the phenotype
the phenotype for a homozygous recessive individual would be different
define monohybrid inheritance
it is the inheritance of a single gene, such as that controlling plant height or seed colour
describe Gregor Mendel’s investigation on pea plants
Mendel chose pairs of contrasting characteristics such as tall or dwarf plants, round or wrinkled seeds and yellow and green seeds
he was fortunate that his choice of characteristics were controlled by single genes and were clear cut and easy to tell apart
why are pea plants a useful choice for inheritance experiments?
they are easy to grow
can self or cross fertilise
can produce flowers and fruits in the same year
give the 5 instructions for genetic crosses
look at the parents: are they homozygous or heterozygous
choose suitable symbols for the alleles (unless already provided)
choose the first letter of the contrasting features if possible
use upper case for dominant and lower case for recessive
clearly label the parents; state their phenotypes, genotypes, and state the gametes produced by each
circle the gametes
remember there will only be one allele for a given gene in each gamete
draw a punnet square to cross the gametes
state the phenotypes and genotypes for the offspring results, with ratios
how would Mendel ensure that offspring produced were only from the parents above (page 5 in booklet)?
pollen transferred by hand - receiving flower could be in a bag to prevent further cross-pollination
remove anthers before they mature so no self-pollination
describe Mendel’s first ‘law of inheritance’
the characteristics of an organism are determined by (factors) alleles, which occur in pairs
only one allele of a pair is present in each gamete
describe continuous variation
the characteristic often has a range of values (usually the bell-shaped normal distribution curve)
controlled by a number of genes (polygenic), e.g. if height is influenced by two or more genes, then they have the potential to grow tall
environmental factors have an effect
describe discontinuous variation
the characteristic has distinct categories controlled by a single gene
environmental factors have no effect
state whether these following characteristics are continuous or discontinuous:
tongue rolling
ear lobes attached/unattached
skin colour
height
hair colour
blood type
(in order)
D
D
C
C
C
D
describe the ‘backcross’/test cross method
it is used in genetics to determine whether a particular dominant characteristic observed in an organism is caused by one or by two dominant alleles (heterozygous or homozygous dominant)
the unknown is always crossed with an individual displaying the recessive phenotype
describe pure breeding
two organisms of the same phenotype that, when bred together, produce offspring with the same phenotype (homozygous)
(example on page 8)
what is codominance?
(page 9 in booklet)
is a condition in which both alleles for a gene are expressed when present in a heterozygote
both alleles are written using a capital letter
what is incomplete codominance?
(page 9 in booklet)
where heterozygous individuals produce a phenotype intermediate of the parental phenotypes
the heterozygous condition is somewhere in between
both alleles are written using a capital letter
what is dihybrid inheritance?
(page 10 in booklet)
involves the inheritance of two unlinked genes (genes found on different chromosomes)
independent assortment of these genes produces recombinants (different allele combinations in the gametes)
describe Mendel’s second law (law of independent assortment)
either a pair of alleles may combine randomly with either of another pair
give two key terms of chi-squared
expected - based on ratios gained through genetic crosses
observed - actual ratios
describe the idea behind chi-squared
genetic crosses give expected offspring ratios
however, when the cross is actually carried out the ratio of offspring produced could be different
what does a chi-squared test work out?
if the expected ratio is significantly different from the observed ratio
define null hypothesis
there is no significant difference between the observed and the expected
describe the use of null hypothesis in chi-squared
if there is no significant difference between the observed and expected ratios then we accept our null hypothesis - any difference is down to chance
if there is a significant difference between the observed and the expected then we reject the null hypothesis - other factors must be having an influence
give the steps on how to use the chi-squared equation: χ² = Σ ((O - E)² / E)
(look at pages 14-15 in booklet)
equation can be solved using a table (sometimes you may need to add columns to the table)
once chi-squared value is worked out, we use the chi-squared table to work out if the value is significant or not
the value for the degrees of freedom is one less than the number of categories (i.e. phenotypes) we’ve used aka: no. of phenotypes - 1
we always compare our value to the value at the 5% significance level (shown usually as 0.05) unless the question says otherwise
if the chi-squared value is less than the value in the table, we accept the null hypothesis, therefore any difference is due to chance
if the chi-squared value is greater than the value in the table, we reject the null hypothesis, therefore other factors are influencing
what are the two types of chromosomes?
autosomes - any chromosome that is not a sex chromosome
sex chromosomes - a chromosome concerned in determining the sex of an organism, typically one of two kinds
in humans, they are similar in one sex (f) and dissimilar in the other (m)
out of the 23 pairs of chromosomes in humans, which are autosomes and which are sex chromosomes?
first 22 are autosomes
last pair are the sex chromosomes
what does it mean when alleles are sex-linked?
some alleles are carried on the x-chromosome
what is the effect on males in sex-linked inheritance?
the male Y chromosome is much smaller than the X and so carries far fewer genes
for most genes carried on the X chromosome in the male there is only one allele which must be expressed
therefore, in the male any recessive genes on the X chromosome will be expressed in the phenotype
if a male with a sex-linked disease had a son, what are the chances of him having the disease?
no chance
the father will only pass his Y chromosome to his son
males can only inherit X-linked genes from their mother
if a male with a sex-linked disease had a daughter, what are the chances of her having the disease?
females can inherit X-linked genes from their mothers or fathers but require 2 recessive alleles for a disorder to show in the phenotype
why might women with a carrier father and unaffected mother want genetic counselling before having children?
to find out if they are carriers
to know the rises to possible sons
and to decide whether to screen embryos for the disorder
describe haemophilia and a possible symptom of it
(page 18 in booklet)
the individual cannot produce enough of one particular blood clotting protein
slow persistent bleeding
describe duchenne muscular dystrophy and a possible symptom of it
(page 19 in booklet)
the dystrophin gene codes for the protein dystrophin, which stabilises cell membranes of muscle fibres
the disease leads to the progressive weakening and wasting of the muscles
why might there be a low number of children who have sex-linked diseases through affected parents?
affected parents might die before they can reproduce or decide they don’t wish to conceive and pass the disease onto offspring
describe linkage
a chromosome contains a linear sequence of genes which are all linked and are usually inherited together
however, if genes are on different chromosomes they are not linked
Mendel’s ratios (3:1, 9:3:3:1) only apply when genes are on different chromosomes - not linked
The genotype AaBb has two genes (A and B) that are found on the same chromosome - linkage
what gametes can be produced by this genotype of crossing over does NOT disrupt the linkage?
AB
ab
The genotype AaBb has two genes (A and B) that are found on the same chromosome - linkage
what gametes would be produced if crossing over did occur - incomplete linkage?
what percentage of crossing over affects linkage?
AB, ab - many of these because they are the parental combinations
aB, Ab - fewer of these because recombination is rare
crossing over disrupts linkage only 5-10% of the time
describe incomplete linkage
crossing over can affect the linkage of genes and leads to incomplete linkage
how likely are genes going to be affected by crossing over in linkage?
if they are further apart on a chromosome
summarise linkage and incomplete linkage
(page 22 in booklet)
linkage:
genes are inherited together
crossing over is very unlikely to separate them as the genes are too close together on the same chromosome
incomplete linkage:
genes are on the same chromosome bat are far apart
genes could be separated due to crossing over and not be inherited together (rare (5-10%))
define mutation
an unpredictable change in the genetic material of an organism
give the two types of mutations
gene mutations - affect single bases within a gene
chromosome mutations - cause changes in the structure or number of whole chromosomes (many genes are affected)
why are mutations important?
they are spontaneous random events that provide an important source of genetic variation
describe mutation rates
rates are low but in organisms with short life cycles and frequent cell division they are more frequent
when do chromosomal mutations most often occur?
during crossing over in prophase I and non-disjunction during anaphase I and anaphase II
give some examples of mutagens
radiation: X-rays, gamma radiation, UV light
chemicals: polycyclic hydrocarbons in cigarette smoke
why is it incorrect to say that mutagens cause mutations?
mutagens do not cause mutations but increased exposure to mutagens increases the rate of mutations occurring
define carcinogens
mutagens that increase the rate of cancer
what genes regulate cell division in humans and what happens when they become mutated?
proto-oncogenes and tumour-suppressor genes
if mutated they form oncogenes
what are oncogenes involved in?
uncontrollable cell division and can lead to cancer
describe benign and malignant tumours
benign - go slowly and do not spread
malignant - made up of cancerous cells and spread
what chemicals does tobacco smoke contain that is harmful to human health?
tar (toxic chemicals)
nicotine (addictive)
carbon monoxide
what does tar contain and what cancer may develop?
carcinogens which target the DN in the cells of the alveoli
if mutation occurs in proto-oncogenes/tumour-suppressor genes, lung cancer may develop
what three things could a change in a single base of a gene result in?
change in DNA base sequence, resulting in a change in triplets/codons for amino acids
different amino acid sequence produced in translation
bonds form in different places and result in a different shaped protein (may not function)
use the enzyme melanin to give an explanation of how a mutation in the gene coding for this enzyme results in a lack of pigmentation in individuals with albinism
changes to the DNA base sequence results in a change in the sequence of amino acids during translation
a mutation in the gene coding for the enzyme could result in the active site of the enzyme changing shape
the enzyme cannot function and no melanin is produced
give an example of a gene mutation and what type of mutation it is
(page 26-27 in booklet)
sickle-cell anaemia
substitution
sickle cell trait also confers some resistance to the malaria parasite
why do you think this is that case?
plasmodium parasites cannot successfully reproduce within the red blood cells because of their sickle shape
describe three possible changes in chromosome mutations
changes in chromosome structure:
errors occur when chromosomes exchange sections of DNA during crossing over at prophase I
changes in whole sets of chromosomes - polyploidy:
if a gamete receives two sets of chromosomes during a failed meiotic division, the gamete will be diploid rather than haploid
changes in chromosome number - nondisjunction:
nondisjunction is a process in which faulty cell division results in one daughter cell getting two copies of a chromosome, whilst one daughter cell gets none
how would fertilisation be affected if polyploidy occurs and where is polyploidy common in?
a triploid zygote (or tetraploid if two diploid gametes fuse)
common in flowering plants such as tomatoes and wheat
if it occurs in animals they will not survive and may be spontaneously aborted or stillborn
what chromosome is affected in down’s syndrome (trisomey 21) and describe how it happens?
chromosome 21
during meiosis in the female ovary, an oocyte with two copies of chromosome 21 is produced and survives
if the oocyte is fertilised, the zygote would have 3 copies of chromosome 21 - two from the mother and one from the father
give some features of down syndrome
open facial features
low muscle tone
some degree of learning disability
explain how a trisomy such as trisomy 26 in mice can occur in a zygote
nondisjunction/homologous chromosomes fail to separate
at anaphase I (in oogenesis)
secondary oocyte has two copies of chromosome 16
third added from sperm at fertilisation
define epigenetics
the control of gene expression by modifying DNA or histone proteins, without affecting the DNA nucleotide sequence
describe epigenetics
evidence shows that the environment can alter the expression of genes by affecting how they are transcribed
these changes are epigenetic - they affect gene expression but not their nucleotide sequence
DNA can be modified post-replication
this changes the ability of a gene to be transcribed in protein synthesis
give two ways DNA can be modified post-replication
addition of a methyl group
modification of histone proteins
describe addition of a methyl group to DNA
a methyl group is added to nitrogenous bases
this prevents these bases being recognised and therefore reduces the ability of the whole gene to be transcribed (expressed)
if transcription does not occur translation will not occur, and the polypeptide and eventual protein will not be produced
describe modification of histone proteins to DNA
these proteins are used to organise the DNA in a chromosome
if the DNA becomes more tightly coiled around the histone proteins - this can prevent gene expression (transcription and translation)
if coiled more loosely it can increase gene expression
what results in different expressions of the same gene within one individual organism?
different epigenetic modifications can occur in cells of the same tissue and in different tissues of the same organism
