Inheritance Flashcards
What are homologous chromosomes?
- two chromosomes that carry the same GENE in the same positions
- same shape
- (same gene, not necessarily allele)
What is meiosis?
- nuclear division
- results in the production of four daughter cells with HALF the chromosome number of the parent cell
- and reshuffled alleles
Describe Prophase 1
- chromosomes condense, become visible
- nuclear envelope disappears
- nucleolus disappears
- centrioles migrate to poles
- microtubules assemble
- homologous chromosomes pair up forming a bivalent
- crossing over occurs at chiasmata
How does crossing over lead to genetic variation?
- occurs in prophase 1
- homologous chromosomes pair up forming a bivalent
- crossing over: exchange of alleles between non-sister chromatids of homologous chromosomes
- this occurs at the CHIASMA (position)
- linkage groups broken
- results in new combinations of alleles
Describe metaphase 1
- bivalents line up along equator of spindle, attached by centromeres
- random alignment
- each pair lines up independently of each other
- 2^n combinations
How does independent assortment lead to genetic variation? (meiosis 1)
- homologous chromosomes line up randomly on equator
- this produces many chromosome combinations
- 2^n combinations
- new combinations of maternal and paternal alleles
What is reduction division?
- only meiosis 1 is a reduction division
- since there was a reduction in chromosome number
- original cell was diploid, but after meiosis 1, 2 cells formed are haploid
- after meiosis 2, 4 haploid cells are produced
What is the gene locus?
- the position of a gene on a chromosome
What is an allele? (3 marks)
- a variety/alternative form of a gene
- determines one form of a characteristic
- occupies same gene locus
- sequence of bases
- can be dominant or recessive
What is a gene?
- a small length of DNA on a chromosome
- which codes for a particular polypeptide
What are the genotypes and phenotypes of an organism?
Genotype: the alleles possessed by an organism (e.g BB)
Phenotype: the observable features of an organism, affected by genes AND environment (e.g Brown coat)
What is a dominant allele?
- An allele that has always has an effect on phenotype if present
- expressed in BOTH homozygous and heterozygous individuals
- e.g BB and Bb both brown coat
What is a recessive allele?
- An allele that only affects phenotype if NO dominant allele is present
- only expressed in homozygote
- e.g bb is white coat
What is codominance?
- codominant alleles both affect the phenotype when both alleles are present
- e.g blood group
- allele A and B = dominant
- allele O = recessive
What is monohybrid inheritance?
- inheritance of one gene
What are F1 and F2 crosses?
F1 cross: offspring resulting from the cross between individuals with homozygous recessive and homozygous dominant genotype
- homozygous recessive x homozygous dominant = heterozygous offspring
F2 cross: offspring resulting from a cross between F1 offspring
- heterozygous offspring x heterozygous offspring
How is a test cross carried out?
- organism showing dominant characteristic
- is crossed with a homozygous recessive organism
- the phenotypes of the offspring can indicate whether the original organism is homozygous or heterozygous
What is a sex-linked gene?
- a gene found on a region of a sex chromosome that is not present on other sex chromosomes
- in humans, most sex-linked genes are found on X chromosome
- sex chromosome: X and Y
What is a carrier?
- an individual who possesses a particular allele as a single copy
- the effect of this allele is masked by the dominant allele
- so the organism will not display the characteristic in its phenotype
- however can be passed to offspring, who can display the characteristic
- females can be carriers, whereas males cannot
What is dihybrid inheritance?
- inheritance of two genes
What is epistasis?
- the interaction of two genes at different loci; one gene may affect the expression of the other
What is autosomal linkage?
- the presence of two genes on the same autosome (any chromosome other than sex chromosome)
- so they tend to be inherited together and do not assort independently
What is parental type and recombinant genotype?
- parental type: offsprings that show the same combinations of characteristics as parents
- recombinant: offspring that show different combinations of characteristics from their parents (due to crossing over in prophase 1)
What is the chi-squared test used for?
- a statistical test that is used to determine whether differences between observed and expected results are a significant
What are the causes of albinism?
- gene: TYR
- albinism is a lack of melanin in skin, eyes and hair
normal process:
- normal gene codes for enzyme called tyrosinase
- tyrosine is converted to DOPA/dopaquinone (by tyrosinase)
- which results in formation of melanin
- in melanocytes
mutant allele:
- mutant allele is recessive
- tyrosinase is not produced/inactive
- affects skin/hair
- only in homozygous recessive people
What are the causes for sickle cell anemia?
- gene: HBB
- HBB codes for amino acid sequence in B-globin polypeptide
mutant allele:
- recessive
- homozygous for mutant allele (HbS)
- CTT replaced with CAT
- glutamic acid to valine
- results in an altered B-polypeptide in haemoglobin
- haemoglobin is less soluble in lower oxygen concentrations
- red blood cells sickle
- so RBC carry less oxygen
- get stuck in capillaries
- clump together and block blood flow
pros:
- protection against malaria
What are the causes of haemophilia?
- gene: F8
- Factor 8 protein plays a role in blood clotting
- synthesized in liver cells
Mutant F8 allele:
- sex-linked gene
- homozygous recessive
- abnormal F8 alleles result in:
- production of abnormal factor 8 protein
- less production of normal factor 8 protein
- no production at all
What causes Huntington’s disease?
Huntington’s is a neurological condition
- brain neuron degeneration
- involuntary movements, uninhibited motor control
Mutant allele: HTT
- DOMINANT
- mutation on chromosome 4
- normal allele has 10-35 repeating CAG units
- dominant allele has extra CAG repeats
- usual onset is middle age
What is a structural gene?
- gene that codes for a protein with a specific function within a cell
What is a regulatory gene?
- codes for a protein that helps control expression of other genes
- e.g repressor protein in prokaryotes, transcription factors in eukaryotes
What genes are present in the lac operon?
regulatory gene
- lac I: codes for repressor protein
structural genes
- lac Z: codes for B-galactosidase (lactase)
- lac Y: codes for permease (allows lactose to enter cell)
- lac A: codes for acetyl transferase
How is gene expression controlled in prokaryotes?
Lactose absent:
- repressor protein binds to operator
- so RNA polymerase unable to bind to promoter
- transcription of structural genes cannot occur
- enzymes and proteins cannot be synthesized
Lactose present:
- lactose binds to repressor protein
- distorting its shape
- so it can no longer bind to operator
- RNA polymerase able to bind to promoter
- transcription occurs
What is a transcription factor?
- a molecule that affects whether a gene is expressed or not
What is an inducible enzyme?
- an enzyme that is synthesized only when a substrate is present
- can be switched on/off
- causes gene expression
e.g B-galactosidase enzyme synthesized when lactose is present
What is a repressible enzyme?
- an enzyme that is normally produced, and whose synthesis is prevented in the presence of an effector
- effector binds to/ activates repressor protein and allows repressor to bind to operator
- end-product inhibition
How is gene expression controlled in eukaryotes?
- Control of seed germination, done by plant hormone called Gibberellin
- influences transcription of amylase enzyme
Gibberellin absent:
- Repressor protein DELLA bound to PIF
- PIF can NOT bind to promoter
- no transcription occurs
Gibberellin present:
- Gibberellin binds to receptor of an enzyme
- this causes the DELLA repressor protein to break down
- PIF CAN bind to promoter
- transcription can occur
- amylase produced, seed germinates
What is the genome?
- ALL the genetic material/DNA present in an organism
- combined nuclear and mitochondrial DNA
