Topic 15: Genotypes And Phenotypes Flashcards
What determines the appearance of an organism?
Expression of genes and influence of environment on gene expression.
What is the phenotype?
Expression of an organisms genotype in its structural, biochemical and physiological characteristics.
What is the genotype?
Refers to both the double set of genetic instructions in a diploid organism and the genetic makeup of an organism at one particular gene locus.
What is homozygous?
Both chromosomes contain same copies of allele for particular trait. Both dominant or both recessive Allen may be inherited.
What is heterozygous
Organism inherits two different allés for particular trait. Dominant allele is expressed.
What is hemizygous?
Genotype of human male relating to gene carried on X or Y chromosome, thus gene only comprises one allele.
Talk about genes located on X chromosome.
- sex linked
- X chromo has more genes on it due to larger size than Y chromosome
- with gene located on sex chromosomes, traits controlled by alleles do not appear equally in both sexes
- females must inherit two copies of allele to be affected whereas males require one only thus males show x-linked traits more
- haemophilia, muscular dystrophy
Talk about Y chromosomes.
Small segments of DNA at ends are homologous to those on ends of X chromosome, allowing sex chromosomes to synapse to form bivalents and then disjoin correctly in Meiosis as they behave as a homologous pair. X linked genes include amely (tooth enamel organisation), and A2F controlling sperm formation.
What is complete dominance?
Occurs where mutually exclusive genetic instructions given, only 1 trait thus can be expressed.
What is dominant?
Allele expressing phenotype in heterozygous and homozygous forms.
What is recessive?
Trait that does not show phenotype unless in homozygous form.
What is a carrier?
Heterozygote that has allele for recessive trait but does not express it.
How do you decide whether a trait is dominant or recessive and why?
Phenotype of heterozygous organism must be identified. Trait that is expressed must be dominant, as dominant trait is expressed in hétérozygotes.
What is co-dominance ?
Both alleles expressed in heterozygous condition. For example, blood type AB is codominant phenotypic expression of alleles A and B in heterozygous condition.
Name environmental factors that impact the expression of genes.
Wind exposure, water availability, acidity, temperature, soil type.
Describe the inherited disorder PKU (phenylketonuria).
- phe hydroxylase enzyme cannot be produced as baby inherited two recessive alleles thus recessive trait expressed
- offspring fed diet with protein including amino acid phe, baby suffers brain damage, becomes mentally retarded
- diet with low levels of phe amino acid, normal phenotype expressed
THUS phenotype due to particular genotype only appears in specific environment, different environment may result in another phenotype.
What are monogenic traits?
- variation occurring in individuals due to action of different alleles of single gene.
- multiple alleles, trait expressed by single gene
What is discontinuous variation?
Members of population classified into distinct non overlapping classes. Quantitative, described in words.
Monogenic traits show discontinuous variation.
What are polygenic traits?
- Caused by action of two or more genes at different loci
- genes involved = polygenes
- often influenced by environmental factors.
What is continuous variation?
Variety of phénotypes, cannot be categorised into distinct groups.
Quantitative values attained. Bell shaped distribution.
As number of polygones involved in expression of single trait increases, number of possible variations in trait also increases.
Describe the four steps of NATURAL SELECTION
- Variation exists in characteristics within species - original changes appear via mutations
- Differential survival: selective pressure selects for favourable characteristics with survival advantage as they can survive better in environment.
- Genetic inheritance: individuals with favourable characteristics survive and reproduce passing characteristics onto subsequent generations which in turn has more offspring
- Frequency of organisms with advantageous characteristics increase in population thus change in population over time
What is epigenetics?
All changes to genes aside from changes to base sequences which bring about phenotypic changes. Epigenetics includes factors that can bring about these changes acting on DNA to permanently turn it on or off.
What are epigenetic factors?
Change how DNA in cells is packaged/labelled.
Genes in segments of DNA tightly packaged = silenced.
Genes in segments of DNA with opening packaging = active in transcribing polypeptide gene products.
Name an example of an epigenetic tag.
- labelling of DNA doesn’t change base sequence BUT can silence or make active genes.
- methyl groups (-CH3) is epigenetic tag added to cytosine base
- active genes = fewer methyl groups
- thus addition of methyl gene changes gene expression by permanently switching genes off
What happens to epigenetic tags once they have attached to base sequences
- Remain for life of the cell, transmitted to all daughter cells.
- Usually not passed to next gen as DNA of fertilised cell cleared of tags
- Epigenetic tags may however be not erased but conserved and passed to subsequent generations.
Explain epigenetic inheritance in terms of cell differentiation.
- embryo cells derived from single fertilised egg via mitotic divisons
- cells develop along different pathways - brain cells, nerve cells etc
- identical génotypes present but different sets of genes active to differentiate
- epigenetic factors produce changes starting stem cells done different developmental paths
Describe epigenetic inheritance in terms of X inactivation.
- somatic cells of normal female embryos 2 copies of X chromosome
- in early embryonic development, one X chromosome inactivated
- epigenetic tag causes inactivation which transmits to all sighted cells produced
- through mitotic cell divisons same X chromosome remains inactive
Describe chemical action in terms of epigenetic inheritance
- ingestion of fungicide vinzoclozin interferes with sperm formation
- pregnant rats injected, male offspring produced reduced numbers of sperm with lower mobility
- defect passed to subsequent generations
- chemical did not mutate DNA, rather change in methylated DNA (increase) observed across 3 gens
