inheritance Flashcards
define gene
a gene is a unit of inheritance located at a particular locus of a chromosome. it is a specific DNA nucleotide sequence which codes for RNA or a polypeptide
define locus
locus is the specific location of a gene on a chromosome. it may contain alternate forms of the gene
define allele
an allele is an alternative form of a gene at a particular gene locus. it is responsible for determining contrasting traits of the same character
all alleles of a gene determine the same character, but each has a unique DNA nucleotide sequence, which may result in different phenotypes.
alleles occr in pairs in a diploid cell although only one of the pair is represented in a gamete
define genotype
genotype is the complete genetic makeup/allelic composition of an organism.
the term is also commonly used in references to the paired alleles carried by an organism that gives rise to a phenotype
define phenotype
phenotype is the physical manifestation of a genetic trait that results from a specific genotype and its interaction with the environment
define wild-type
wild-type is the most common allele/phenotype in nature
define true-breeding
a true-breeding organism gives rise to all offspring of the same phenotype i.e. the organism is homozygous
define carriers
a carrier is an organism that has inherited a recessive allele for a genetic trait or mutation but does not display that trait or show symptoms of the disease
define homozygous
homozygous is the condition in which the alleles of a gene pair, in diploid condition, are identical. an organism with this condition is known as a homozygote, and referred to as true- or pure-breeding. all gametes produced by this organism will carry the same allele
define heterozygous
heterozygous is the condition in which the alleles of a gene pair in diploid condition are different. an organism with this condition is known as a heterozygote. 50% of its gametes will carry 1 allele and the other 50% will carry the other allele
define dominant allele
dominant alleles produce their effects in both homozygous and heterozygous conditions. (one copy of the allele is sufficient to cause the organism to express the phenotype it encodes) a dominant allele masks the influence of the recessive allele.
an organism homozygous for a dominant allele is homozygous dominant
define recessive allele
recessive alleles produce their effects only in homozygous conditions. i.e. the recessive allele is only able to express itself in the absence of the dominant allele
an organism homozygous for a recessive allele is known as homozygous recessive
how is genotype linked to phenotype?
- alleles, which are different forms of a gene, are transcribed to form mRNA and translated into different polypeptide chains & proteins, which may affect different metabolic pathways, resulting in the formation of different phenotypes
- each allele thus specifies/codes for the specific condition of a particular protein which directly determines a trait
- a genotype is the genetic makeup of an organism, which refers to the paired alleles that produces a phenotype, which is a measurable/distinctive character
eg: stem height of pea plants
- in a pea plant with tall stem, its genotype consists of either one or two copies of the dominant allele of the gene for stem height -> so the plant displays a phenotype of tall stem, as a result of the presence of a protein which contributes to the growth of stem
- if the genotype lacks a dominant allele/consists of 2 copies of recessive alleles of the gene for stem height, the plant displays a phenotype of dwarf stem as a result of the lack of protein which contributes to the growth of stem
what is monohybrid inheritance?
monohybrid inheritance is when a single character is controlled by a single gene (a pair of alleles)
3:1 phenotypic ratio in F2 progeny
what is mendel’s first law of segregation?
mendel’s first law of segregation stateehat during the formation of gametes, the paired alleles segregate randomly so that each gamete receives one allele or the other with equal likelihood
what is a test cross?
a test cross involves crossing an organism of (dominant phenotype but) unknown genotype with a homozygous recessive individual. the phenotype of the offspring indicayes the genotype of the organism tested
what is incomplete dominance?
incomplete dominance is a condition where neither of the 2 alleles is completely dominant to the other, so that the heterozygote has a phenotype which is intermediate
1:2:1 phenotypic ratio in F2 progeny
what is codominance?
codominance is the phenomenon in which both alleles are equally expressed in the phenotype of the heterozygote. the heterozygote simultaneously expressesthe phenotypes of both types of homozygotes
1:2:1 phenotypic ratio in F2 progeny
what is multiple alleles?
if a gene controlling a characteristic has three or more alleles, the alleles are called multiple alleles. there is no fixed ratio during the inheritance of genes with multiple alleles
what are lethal genes?
mutations which lead to a non-functional gene product can sometimes be tolerated in the heterozygous state, but not in the homozygous state, leading to lethality.
eg for the coat colour in mice, homozygous yellow (YY) mice die because the YY genotypes leads to extensive deletion of genetic material characterizing the Y allele extends into the coding region of an adjacent gene that is critical to embryonic development, rendering it non-functional -> so the mice with YY genotype die
2:1 phenotypic ratio in F1 progeny
1:2:1 genotypic ratio but 1 dies, so 2:1 left ://
what is dihybrid inheritance?
the inheritance of 2 pairs of contrasting characters in each cross. the 2 characters are controlled by 2 genes located on 2 gene loci on 2 different chromosomes.
dihybrid cross is a genetic cross involving 2 characters in which the parents possess different forms of each character
9:3:3:1 phenotypic ratio in F2 progeny
what is mendel’s second law of independent assortment?
mendel’s second law of independent assortment states that the segregation of one pair of alleles is independent of the segregation of other pairs
NOTE: only for unlinked genes (genes on different chromosomes)
i.e. whichever allele is received by a gamete does not influence the outcome of segregation of any other pair
independent assortment stimulates that all 4 combinations (AB, Ab, aB, ab) will be formed with equal probabilities for a cell with AaBb genotype
what will be the outcome of a test cross, where an organism that expresses 2 dominant traits, with unknown genotype, is crossed with one that is homozygous recessive for both gene loci?
dihybrid test cross
- AABB x aabb -> all AaBb
- AaBb x aabb -> 1 AaBb:1 Aabb:1 aaBb:1 aabb
- AABb x aabb -> 1AaBb:1 Aabb
- AaBB x aabb -> 1 AaBb:1 aaBb
what is sex linkage?
sex linkage refers to the carrying of genes on the sex chromosomes. the genes are mostly located on the X chromosome as the X chromsome is longer than the Y chromosome and carries more genes compared to the Y chromosome. X chromosomes contains many loci that are required in both sexes, whereas the Y chromosome contains only a few genes
so genes located on the sex chromosomes, mainly the x chromosomes, are known as sex-linked genes/ X-linked genes as they follow the transmission pattern of the X chromosome
why are males more susceptible to sex-linked disorders?
- males only have one sex-linked gene locus (bc only have 1 X chromosome) -> they are neither homozygous nor heterozygous for sex-linked loci, but instead is hemizygous
- as males possess only a single X chromosome, whatever allele present on the X chromosome of males will be directly expressed in the phenotype, whether or not the allele is dominant or recessive.
- so X linked disorders mainly affect males as they are hemizygous
for recessive X-linked disorders,
- males only need one copy of the defective recessive allele to have the disorder, whereas females require 2 copies of the defective recessive allele
what are examples of human X-linked recessive disorders?
- haemophilia
- red-green colour blindness
- duchenne muscular dystrophy
what are some principles regarding sex-linked inheritance?
- an affected father will transmit the recessive X-linked allele to all daughters, whom become carriers (i.e. hetereozygous for the trait), but not to any sons, because his son will inherit his Y chromosome only and the X chromosome from his mother
- if a carrier female has a child with a normal male, there is a 50% chance each daughter will be carrier and a 50% chance each son will have the disorder
- if a carrier female has a child with an affected male, there is a 50% chance that each child, regardless of sex, will have the disorder. daughters who don’t display the recessive phenotype will be carriers, whereas males without the disorder will be completely frre of the recessive allele
what is a reciprocal cross?
a reciprocal cross is a pair of crosses in which the traits of the 2 parents are reversed. eg cross 1: red eye female, white eye male. cross 2: white eye female, red eye male
a reciprocal cross can be conducted to discern if a trait is carried on a sex chromosome or on an autosomal chromsome
how do the results of a reciprocal cross allow us to conclude whether a trait is X-linked or not?
if the trait is X-linked, the reciprocal crosses will yield non-identical results
if the trait is autosomal, the reciprocal crosses will yield identical results
key tips in pedigree analysis
- if an offspring displays a trait although both parents lack the trait, it means the trait is recessive
- if both parents are affected an all children are affected, the trait is recessive
- if traits skip generations, the trait is recessive
- if all offspring of unaffected parents are unaffected, the trait is dominant
- if traits do not skip generations, the trait is dominant
- if most affected individual are male, the trait is X-linked
- if all daughters of an affected father are affected, the trait is X-linked and dominant (assuming mother is normal)
what are linked genes?
linked genes are genes that control different characters and are situated on the same chromosome at different loci
linked genes will not produce normal dihybrid inheritance ratios of 9:3:3:1 (double heterozygote cross) or 1:1:1:1 (test cross)
why are no recombinant phenotypes formed in complete linkage?
- the 2 pairs of genes are located very close together on the same chromosome and are closely linked together
- no crossing over occurs between the 2 gene loci so they are transmitted together as a unit into the same gamete, resulting in only parental gametes
- the 2 phenotypes formed are both recombinant phenotypes
why can recombinant phenotypes be formed in incomplete linkage, but in smaller proportion?
- the genes are located some distance apart on the same chromosome, and thus can be separated when crossing over occurs during meiosis
- during prophase I of meiosis, crossing over may occur between the 2 gene loci, so there will be genetic recombination leading to a new combination of alleles in gametes
- the separated chromatids eventually end up in separate gametes, and after fertilisation, give rise to new combinations of alleles in the offspirng, which are called recombinants
- since crossing over is a random process, majority of offspring produced will show parental allele combinations and hence parental phenotypes and a minority of recombinant allele combinations, and hence recombinant phenotypes
the probability of crossing over occurring between 2 gene loci increases with increasing distance between the 2 gene loci
how can linkage be detected using a test cross?
conduct a test cross between a double heterozygous individual with a double homozygous recessive individual
if the genes are present on different chromosomes,
- offspring of 4 different phenotypes should be produced with a ratio 1:1:1:1
if the genes are completely linked,
- only 2 phenotypes are produced
- offspring should consist of the 2 parental phenotypes with a ratio 1:1, with no recombinant phenotypes
if the genes are incompletely linked
- 4 different phenotypes are produced, 2 parental and 2 recombinant
- there will be a larger percentage of parental phenotypes & smaller percentage of recombinant phenotypes
what is coupling & repulsion arrangement in linked genes?
only applicable for heterozygotes
coupling is when 2 dominant alleles are on 1 chromosome, and the 2 recessive alleles are on the homologous partner. AB/ab
repulsion is when the dominant allele is linked with a recessive on one chromsome. Ab/aB
what is crossover value (COV)/recombination frequency (RF)?
COV & RF are also known as the percentage of crossing over.
COV/RF = no. of recombinants/total no. of offsprin x100
- the chance of crossing over occurring betwen 2 linked genes is proportional to the distance between them
- if the 2 linked genes are far apart on a chromosome, there is a greater statistical chance that crossing over will separate them than if they were closer -> greater proportion of recombinants
- so the distance between genes can be determined by the proportion of recombinants
less recombinants = genes are closer
what is phenotypic variance?
phenotypic variance (Vₚ) of a population has two components, genotypic variance (VG) and environmental variance (VE)
VE is most easily determined when studying homozygous organisms, as VG = 0
VG is due to genotype, and includes the effects of additive genes, dominant genes & epistasis
how is genetic variation introduced in a population?
gene mutation
- an inheritable change in nucleotide sequence of DNA, which occurs at a single gene locus on a chromosome, resulting in the formation of new alleles
- gene mutation can occur by deletion (removing 1 or more nucleotides), insertion (adding 1 or more nucleotides) or substitution (replacing one nucleotide with another)
chromosomal mutation
- a change in the structure of a chromosome OR
- a change in the number of chromosomes
meiosis/sexual reproduction
- crossing over of non-sister chromatids of homologous chromosomes during prophase I of meiosis, resulting in new combinations of paternal & maternal alleles in each chromatid
- independent assortment of homologous chromosomes during metaphase I, resulting in random distribution of maternal & paternal chromosomes in each gamete
sexual reproduction
- the fusion of 2 haploid gametes to form a diploid zygote restoring the diploid number is random, resulting in genetic variation. there is also random mating between individuals in a population
genetic variation from meiosis & sexual reproduction results in new COMBINATIONS of existing alleles
only MUTATIONS result in NEW ALLELES
using coat colour in himalayan rabbits as an example, how does temperature affect phenotype? (LO)
- himalayan rabbits with white body, black ears, nose, feet and tail are homozygous for the cʰ allele of the tyrosinase gene
- the tyrosinase gene codes for a heat-sensitive form of the enzyme tyrosinase, which is needed for melanin production, resulting in black fur
- heat sensitive tyrosinase is active only when temperature is below 33C
- only in parts of the body that are cool enough, eg in extremities (nose, ears, feet, tail) does black fur grow
experiment: himalayan rabbit with white body, black ears, nose, feet & ears had the fur on the back shaved, and an ice pack fixed on the shaved region. when the ice pack is left in position for weeks and kept cold, black fur, instead of the original white fur, begins to grow below the ice pack
using honey bees as an example, how does diet affect phenotype? (LO)
- bee colonies have 3 types of bees: queen, drones & workers
- drones are males and develop from unfertilized haploid eggs
- the queen & workers are females and develop from fertilised diploid eggs
- although the queen & workers have the same amount of genetic material, they are phenotypically different
- this difference is due to the diet of be larvae. after hatcching, all the larvae are initially fed with royal jelly. larvae destined to be worker bees are switched to a diet containing honey and pollen, while larvae destined to be the queen are fed with royal jelly
- the high protein content in royal jelly stimulates the formation & maturation of the female reproductive system
define non-epistatic gene interactions
2 independently assorting genes may interact to influence a single character
same ratio as dihybrid (9:3:3:1), but dihybrid is when 2 characteristics are controlled by 2 genes but gene interaction is when 1 characteristic is controlled by 2 or more genes
define epistasis
epistasis is inferred when the expression of an allele of one gene suppresses/inhibits the expression of alleles of a different gene (the hypostatic gene).
when 2 or more genes influence one characteristic, an allele at one gene locus may have an overriding effect on the genotype at a different locus
define recessive epistasis
for recessive epistasis, 2 recessive alleles at the epistatic gene locus will suppress/inhibit the effect of either allele of the hypostatic gene at a different locus
phenotypic ratio of 9:3:4 with a double heterozygous cross
9 (A-B-) : 3 (A-bb) : 4 (3 aaB- + 1 aabb)
aa suppresses alleles B/b
define dominant epistasis
for dominant epistasis, one dominant allele at the epistatic gene locus will suppress/inhibit the effect of both alleles of the hypostatic gene at a different locus
phenotypic ratio of 12:3:1 with a double heterozygous cross
12 (9 A-B- + 3 AAb-) : 3 (aaB-) : 1 (aabb)
define duplicate recessive epistasis
for duplicate recessive epistasis, 2 recessive alleles at either of the 2 gene loci will suppress/inhibit the effect of the dominant allele at the other locus
phenotypic ratio of 9:7 with a double heterozygous cross
9 (A-B-) : 7 (3 aaB- + 3 A-bb + 1 aabb)
what is variation?
variation describes the recognisable differences in characteristics between organisms of the same natural population or species
how do continuous & discontinuous variation differ in terms of observable phenotype?
discontinuous variation
phenotypes are definite and clear cut, and can be divided into distinct groups, with discrete phenotypic classes observed. intermediates are not observed
continuous variation
phenotypes are not clear cut and cannot be divided into distinct contrasting group, with a range of phenotypes observed. intermediates are observed
how do continuous and discontinuous variation differ in terms of the number of genes controlling phenotypic variation?
discontinuous variation
variation is controlled by a single or a few gene(s). this gene may have 2 or more alleles
continuous variation
variation is controlled by the combined effect of multiple genes, known as polygenic inheritance
gene act on the phenotype in an additive manner. effects of single genes are too slight to be detected.
the combined effect of these genes can produce individuals of infinite phenotypic varieties
how do continuous & discontinuous variation differ in terms of the effect of the environment on phenotype?
discontinuous variation
there is little or no environmental effect on the phenotypic expression of the gene(s)
continuous variation
phenotypes can be modified by the cumulative effect of varying environmental factors acting on the different genotype
although the genotype of an organism is determined at the time of fertilisation, the degree of expression allowed to this genetic potential hinges on environmental factors during the development of the organism
how do discontinuous and continuous variation differ in terms of mode of phenotypic measurement
discontinuous variation
phenotypic measurements are normally represented on bar graphs
measurement is qualitative, analysed by making counts & ratios
continuous variation
phenotypic measuremens form a normal distribution curve that has a continuous range of intermediate forms between 2 extremes. these are normally represented on histograms
measurement is quantitative; statistical analyses give estimates of population parameters such as the mean and standard deviation
what causes genetic variation in asexually reproducin organisms?
DNA replication is asexually reproducing organisms is so highly accurate and almost error-free, that there is little possibility of variation in the genotype. any apparent variation between asexually reproducing organisms is almost always the result of environmental influences
does NOT generate new alleles, only new COMBINATIONS of alleles
what causes genetic variation in sexually reproducing organisms?
- crossing over between non-sister chromatids of homologous chromosomes during prophase I of meiois
- independent assortment of bivalents at the metaphase plate during metaphase I of meiosis
- random fertilisation producing a variety of combinations of alleles
does NOT generate new alleles, only new COMBINATIONS of alleles
new alleles are generated by mutations (for both asexual & sexual), which refer to a change in the structure of a gene or in the structure/number of chromosomes. only mutations that occur during the formation of gametes can be inherited, producing distinct differences between individuals and fors the basis of discontinuous variation
what is the chi-squared test?
the chi-squared test is a statistical test for the significance of data that consists of discontinuous/discrete variables
step-by-step solution of chi-squared test questions?
1 - state the hypotheses
- H₀: there is no significant difference between the observed and expected ratios. (it assumes that any differences are due to chance)
- Hᴀ: there is significant difference between the observed and expected ratios. (it assumes that differences are not due to chance)
2 - calculate the expected number of individuals for each phenotypic class, using the expected phenotypic ratios
3 - calculate the chi-squared value, using the sum of the square of the difference between expected & observed over the expected numbers (formula will usually be given)
4 - compare the calculated chi-squared value against a chi-squared probability table. take the level of significance to be 0.05 unless stated otherwise. degree of freedom (df) = n-1, where n is the number of phenotypic classes.
5 - conclude
- if x²calc > x²crit, the probability that chance alone is the reason for the difference between observed & expected results is less than 5%. the deviation is significant, so reject H₀ in favour of Hᴀ
- if x²calc < x²crit, the probability that chance alone is the reason for the difference between observed & expected results is more than 5%. the deviation is not significant, so do not reject H₀.
