gene
unit of inheritane located at a particular locus of a choromosome. it is a specific DNA nucleotide sequence which codes for RNA or a polypeptide
Locus
The specific location of a gene on a chromosome.
(plural: loci)
Allele
- An alternative form of a gene at a particular gene locus.
- 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 occur in pairs in a diploid cell although only one of the pair is represented in a gamete
dominant vs recessive alleles
dominant alleles:
- Produce their effects in both homozygous and heterozygous condition
- one copy of the allele is sufficient to cause the organism to express the phenotype it encodes, e.g. allele T causes the plant to be tall in either TT or Tt condition.
- A dominant allele masks the influence of the recessive allele.
- An organism homozygous for a dominant allele is known as homozygous dominant.
Recessive alleles:
- Produce their effects only in homozygous condition, e.g. allele t causes the plant to be short only in the tt condition.
- An organism homozygous for a recessive allele is known as homozygous recessive.
codominace
- phenomenon which both alleles are equally expressed in the phenotype of the heterozygote.
- heterozygote simultaneously expresses the phenotypes of both types of homozygotes
eg in cows, heterozygote with roan coat consist of a mixture of red and white hairs
homozygous vs heterozygous
Homozygous:
- Condition in which the alleles of a gene pair in diploid condition are identical, e.g. TT or tt, where T is the dominant allele for tall stem and t is the recessive allele for short stem.
- 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.
Heterozygous
- Condition in which the alleles of a gene pair in diploid condition are different, e.g. Tt.
- An organism with this condition is known as a heterozygote. 50% of its gametes will carry one allele while the other 50% will carry the other allele.
phenotype vs genotype
Genotype:
- The complete genetic makeup / allelic composition of an organism.
- The term is also commonly used in reference to the paired alleles carried by an organism that give rise to a phenotype.
Phenotype
- The physical manifestation of a genetic trait that results from a specific genotype and its interaction with the environment.
linkage
what is the link between genotype and phenotype?
- Alleles, being alternative or different forms of a gene, are transcribed to form mRNA and translated into different polypeptide chains and proteins which may affect different metabolic pathways resulting in the formation of different phenotypes
- Each allele thus specifies or codes for the specific 3D conformation 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 or distinctive character
monohybrid inheritance
it is the inheritance of a single character of contrasting traits (e.g. tall vs short stem)
- character is controlled by a single gene (a pair of alleles)
what r mendel’s postulations
- alleles come in pairs
- In the monohybrid cross involving tall and dwarf plants, a specific allele, T and t exists for each trait respectively.
- Each diploid individual receives one allele from each parent. - dominance or recessiveness
- in Tt, recessive allele is masked by dominant allele, and is only only able to express itself in the outward appearance of the plant
in the absence of the dominant allele, - segregation:
Mendel’s first law of segregation: During formation of gametes, paired alleles segregate randomly so that each gamete receives one or the other with equal likelihood
what is a test cross (monohybrid inheritance)
purpose: involves crossing organism of the dominant phenotype but unknown genotype with homozygous recessive individual, phenotype of the offspring indicates genotype of the organism tested
- if organism displaying dominant phenotype is homozygous, all offspring will express the dominant phenotype
- if organism if heterozygous, about ahlf the offspring will express the dominant phenotype while the other half expresses the recessive phenotype in a 1:1 ratio
what is test cross (dihybrid inheritance)
crossing organism that expresses two dominant traits but unknown genotype with an individual which is homozygous recessive for both gene loci (double homozygous recessive)
what is a lethal gene
lethal allele: one that causes death is organism is homozygous for allele
dominant lethal gene:
expressed in homozygotes and heterozygotes, rarely detected due to rapid elimination from both populations
recessive lethal gene
eg Coat colour in mice is linked to a lethal gene
- mutant allele Y is dominant to wild type agouti allele y
- gene is recessive allele as only with 2 YY, it is lethal
- heterozygous: yellow coat
- cross breeding yellow mice (Yy) –> 2 yellow fur (Yy): 1 agouti fur (yy) cuz homozyogus yellow (YY) mice die
what is a dihybrid cross
genetic cross involving two characters in which the parents possess different forms of each character. the two characters are controlled by two genes (2 pairs of alleles) located on two gene loci on two different chromosomes (unlinked genes)
what is mendel’s second law of independent assortment
segregation of one pair of alleles is independent of segregation of other pairs
this means:
- each F1 gamete receives either a R or r allele AND a Y or y allele
- all four combinations (RY, Ry, rY, ry) formed with equal probabilities
note: independent assortment occurs only if genes are not located on same chromosome (aka not linked genes)
how do 23rd chromosome differ in males and females and how do they determine sex?
females are homogametic: produces one type of gamete, all carry X chromosome
males are heterogametic: produces two types of gametes, half carry X, half carry Y
in offspring, sex is determined by sperm that fertilises egg cell:
egg cell may be fertilised by sperm carrying an X chromosome: zygote has two X chromosomes, female
egg cell fertilised by sperm carrying Y chromosome: zygote has XY, male
what are sex-linked genes / X-linked genes
genes located on sex chromosomes
X chromosome contains many loci that are required on both sexes, hence genes are mostly located on X chromosme
why do human sex-linked inheritance mainly affect males?
As a male has only one sex-linked gene locus (1 copy of X chromosome instead of 2 in females), he is hemizygous (half) for every sex-linked locus
* single X chromosome, whatever allele present on the X chromosome of males will be directly expressed in the phenotype, whether or not if the
allele is dominant or recessive
what are some principles regarding sex linked inheritance
- affected father will transmit the recessive X-linked allele to all daughters but not to any sons because his son will inherit father’s Y chromosome only and X chromosome from mother.
- If the mother is homozygous dominant, the daughters are heterozygous and are carriers of the mutation
- carrier female x normal male: 50% chance that each daughter will be a carrier, 50% chance that each son will have the disorder
- carrier female x affected male: 50% chance that each child born to them will have the disorder, regardless of sex
how does a reciprocal cross work
reciprocal cross can be conducted to discern if a trait is carried on a sex chromosome (X-linked) or on an autosomal chromosome.
A reciprocal cross is a pair of crosses in which the traits of the two parents are reversed.
if results are non-identical in the two crosses, it involves X linked traits, whilst if resutls are identical, cross involves autosomal traits
why do X-linked reciprocal crosses reuslt in non identical results?
- the male transmits the X chromosome only to his female offspring,
- while the female transmits an X chromosome to both male and female offspring.
- Since the male parent does not transmit the X chromosome to his sons, he does not contribute to
the appearance of their X-linked phenotypes.
what are some characteristics of autosomal recessive inheritance
Recessive because:
1. If both parents are affected (aa x aa), all children should be affected
2. In most cases, when unaffected people mate with affected individuals, all children are unaffected.
3. When at least 1 child is affected (means unaffected parent is heterozygous), approximately half the children should be affected.
4. Unaffected parents (heterozygous at gene loci) can produce affected individuals.
5. Trait often skips generations.
Autosomal because:
1. Both males and females are affected with equal probability.
whilst for dominant:
- if both parents are affected (Aa / AA), not all children will be affected (due to possibility of aa genotype which is not affected)
Characteristics of an autosomal dominant inheritance
Dominant because:
1. Unaffected parents (aa) should not have affected children.
2. Trait should not skip generations, i.e. affected individuals must have affected parent(s).
3. When an affected heterozygous (Aa) individual mates with an unaffected (aa) individual, 50% of their offspring should be affected (also shows that affected individual is heterozygous).
Autosomal because:
1. Both males and females are affected with equal probability.
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Carbohydrates28
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Proteins48
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Enzymes30
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Lipids25
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Cell Structure27
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DNA Structure and replication38
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Eukaryotic gene expression49
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Organisation of Eukaryotic genome31
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control of eukaryotic gene expression17
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Gene and Chromosomal Mutations28
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cancer32
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Genetics of Virus26
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Cell signalling24
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Cell membrane33
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Cell and nuclear division46
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stem cells19
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energetics 1: photosynthesis39
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energetics 2: cellular respiration19
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Inheritance40
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Bacteria23
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inheritance II10
