Inheritance Flashcards
1
Q
What are homologous chromosomes?
A
- two chromosomes that carry the same GENE in the same positions
- same shape
- (same gene, not necessarily allele)
2
Q
What is meiosis?
A
- nuclear division
- results in the production of four daughter cells with HALF the chromosome number of the parent cell
- and reshuffled alleles
3
Q
Describe Prophase 1
A
- 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
4
Q
How does crossing over lead to genetic variation?
A
- 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
5
Q
Describe metaphase 1
A
- bivalents line up along equator of spindle, attached by centromeres
- random alignment
- each pair lines up independently of each other
- 2^n combinations
6
Q
How does independent assortment lead to genetic variation? (meiosis 1)
A
- homologous chromosomes line up randomly on equator
- this produces many chromosome combinations
- 2^n combinations
- new combinations of maternal and paternal alleles
7
Q
What is reduction division?
A
- 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
8
Q
What is the gene locus?
A
- the position of a gene on a chromosome
9
Q
What is an allele? (3 marks)
A
- 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
10
Q
What is a gene?
A
- a small length of DNA on a chromosome
- which codes for a particular polypeptide
11
Q
What are the genotypes and phenotypes of an organism?
A
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)
12
Q
What is a dominant allele?
A
- 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
13
Q
What is a recessive allele?
A
- An allele that only affects phenotype if NO dominant allele is present
- only expressed in homozygote
- e.g bb is white coat
14
Q
What is codominance?
A
- codominant alleles both affect the phenotype when both alleles are present
- e.g blood group
- allele A and B = dominant
- allele O = recessive
15
Q
What is monohybrid inheritance?
A
- inheritance of one gene
16
Q
What are F1 and F2 crosses?
A
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
17
Q
How is a test cross carried out?
A
- 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
18
Q
What is a sex-linked gene?
A
- 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
19
Q
What is a carrier?
A
- 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
20
Q
What is dihybrid inheritance?
A
- inheritance of two genes
21
Q
What is epistasis?
A
- the interaction of two genes at different loci; one gene may affect the expression of the other
22
Q
What is autosomal linkage?
A
- 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
23
Q
What is parental type and recombinant genotype?
A
- 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)
24
Q
What is the chi-squared test used for?
A
- a statistical test that is used to determine whether differences between observed and expected results are a significant
25
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**
26
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
27
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**
28
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**
29
What is a **structural** gene?
- gene that **codes for a protein with a specific function** within a cell
30
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
31
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**
32
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
33
What is a transcription factor?
- a molecule that affects whether a gene is expressed or not
34
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
35
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**
36
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
37
What is the genome?
- ALL the genetic material/DNA present in an organism
- combined nuclear and mitochondrial DNA
