Chapter 24 - Variation Flashcards
What is variation?
The differences between members of a species resulted from genetics (fertilisation/mutation)and the influence of the environs first
What are the main sources of Variation?
- Meiosis (sperm and egg)
- random assortment
- crossing - over
- non-disjunction - Random Fertilisation
- Epigenetic factors
- Mutations - genes or chromosomes
What are the benefits to Variation?
The greater the variation, the more likely it is that some individuals can survive a CHANGING ENVIRONMENT
Benefit in the event of a disaster (disease, chemical exposure, climate change, famine, drought) - some service and pass advantageous genes to their offspring
Genetic Variation is essential for Natural Selection
What are the two examples of variation in humans?
- Basic skin colour - inherited from parents, but if light-skinned, exposure to the sun/other environmental factors may result in modification to colour
- Exposure to infectious disease or have been vaccinated — developed immunity to the disease and the fire influences changes in the characteristics (immune system)
Summary of Mendel’s findings
Characteristics he was studying were inherited INDEPENDENTLY
Eg. round vs. wrinkled factory can exits in all coloured (green vs. yew looks peas ect)
Each trait will not affect the other as they are independent
Explain the principle of INDEPENDENT ASSORTMENT by Mendel
Since inheritance of his research/study characteristics were inherited independently he decided that each individual must have 2 factors (GENES) for a characteristic and DURING the formation of Gametes (sperm and egg cells) - when the pairs of factors separate
What happens during fertilisation?
When 2 Gametes unite at fertilisation, the resulting cell has a combination of genes that is different from either parents.
Gametes are formed via a type of cell division - meiosis
- each cell has 2 sets of chromosomes
1x from the male parent (paternal chromosome)
1x from the female parent (maternal chromosome)
Thus the chromosomes exits in Homologous pairs
What are homologous pairs?
The genes on one member of the pair control the same characteristics as the genes in the other member of the pair
Explain Random Assortment
- During the 1st meiotic division the homologous pairs separate at a random.
- One member if each pair moves to one pole of the cell, while the other member of the pair moves to the opposing pole
- Results in 23 pairs of chromosomes at each pole of the cell
- Random assortment takes place for each of the 23 chromosomes pairs of human chromosome - that means any single egg (human) Receive one of the 2 possible chromosome 23x
- The same random assortment goes in for each sperm produced.
- Thus when sperm fertilised the egg the resulting fertilised cell contains a combination of genes arranged in another order that has probably never occurred before and probably never will.
Important feature when chromosome separate in the first meiotic division.
When chromosomes separate in the first meiotic division they do so INDEPENDENTLY.
The way one pair of chromosomes separate is unaffected by the way any of the after pairs seperate
Eg. Copy if Chr 1 that egg cell receives in no way influences which if the possible copies of chr. 5 it gets.
What is the total number if possible chromosome combinations for eggs
2^(23)
~ 8.4 million
Define Crossing Over
A situation when homologous chromosomes pair during the first diving if meiosis, the chromatids may get entangled with another
What is CHIASMA (pl CHIASMATA)
The point where 2 chromatids Cross
Explain crossing over
The alleles in a particular chromatids tend to be inherited as a group, because which ever gamete receives that chromosome at meiosis will receive all alleles in the chromosomes.
The alleles in a chromosome are said to be linked
RECOMBINATION- when crossing over occurs, the chromatids may break and reattach fo e chromatids from a different chromosome.
When this occurs the result is a new combination of allles along the chromosome
If a garment contains a chronometer with a recombined set of alleles is involved in fertilisation, the resulting offspring will have the new combination of alleles
This combination of alleles is not the same as that possessed by either parents.
Remember that when chromosomes become visible at meiosis
They are already duplicated and each consists of a pair of chromatids joined at the centromere.
Explain the movement of alleles.
Tracking the movement if alleles during crossing over helped scientists determine roughly how far apart 2 genes were on a chromosome.
Because there is more chances for a break to occur between the alleles of 2 genes that lie far apart, it is more likely that one allele will stay on the original chromosome while the other crosses over.
Therefore alleles that lie further apart are more likely to end up on 2 different chromosomes.
On the other hand alleles that lie very close together are far less likely to be separated by a break and to cross over.
By examining the number of offspring with recombination of alleles, we are able laboriously to map the order of genes on a chromosome.
Today DNA analysis techniques are used to determine the genome.
What is non-disjunction
During the first division of meiosis - homologous chromosomes pair and then separate
-Sometimes 1 or more of the chromosome pairs may fail to separate when the cell divides
These situations are called NON -DISJUNCTION
They result in one of the daughter cells receiving an extra chromosome and the other daughter cell lacking that chromosome.
Explain Non-junction in humans
- In humans non-disjunction occurs in one if the chromosomes pairs during meiosis, the resultant Gametes will have either 24 chromosomes or 22 chromosomes instead of the normal 23.
- After fertilisation with a normal gamete from the opposite sex, the zygote produced will have either 47 or 45 chromosomes respectively.
- This will produce quite unexpected characteristics in the offsprings.
- Usually such changes to the chromosome number cause severe and distinctive birth defects and miscarriage often occurs early in pregnancy.
What is Trisomy? Example of defect caused by this.
Trisomy is a condition in which an individual inherits an extra copy of a chromosome - 3 copies instead of the normal 2.
One Such chromosomal defect that occurs relatively frequently, especially in children of older mothers is DOWN SYNDROME (or trisomy 21)
What happens if an individual has Down Syndrome?
In this disorder an extra copy of Chromosome 21 results in a
- characteristic facial appearance
- variable degrees of mental retardation
- physical abnormalities
What are the cases where trisomy occurs in other chronometer of the body?
- Extra chromosome 13 produces individuals with mental retardation, small head, an extra finger on each hand, cleft palate and/or cleft lip and malformations in the ear
- Extra chromosome 18 results in the individual suffering from mental retardation and defects in the eyes, ears, hands and head.
- Down syndrome
What is Monosomy
Monosomy is where an individual is missing a chromosome - they have only one copy instead if the normal 2.
- usually results in server malformations and often miscarriage
Example -Turner syndrome;
- condition that affects only females, results when one of the X chromosomes (sex chromosomes) is missing or partially missing. -medical and developmental problems, including short height, failure of the ovaries to develop and heart defects.
List the Non disjunction Disorders
- Turners Syndrome
- Down Syndrome
- Cri -du - chat
- Klinefelter syndrome
What is Partial Trisomy?
Partial Trisomy occurs when part of an extra chromosome is attached to one of the other chromosomes.
Partial trisomy 21 can result in many of the of Down syndrome
Explain Turners syndrome
Monosomy XD
Affects females 1:500
- missing Sex chromosome
- 50% of theses conceptions are aborted
- diversity of symptoms
- short stature
- infertility
- heart abnormality
Explain Cri - Du -Chat
Partial Monosomy
- missing part of chromosome 5
High pitch cry at birth
Round face
Small head
Eyes spaced wide apart
Explain Klinefelter
Trisomy - XXY (males)
Extra sex chromosome Subtle symptoms Taller than average height Infertility Small poorly functioning testes Breast growth
Risk with maternal age
Explain Random Fertilisation
There is no way of determine exactly which sperm cell will fertilises any particular egg.
RANDOM FERTILISATION in which any kind of sperm from the male parent can fuse with any kind of egg from the female parent
- therefore another source of variation that occurs as a result of sexual reproduction.
- fertilisation relies completely up to CHANCE
- thus more variability in the offspring is introduced, depending on which sperm or egg is successful in fertilisation.
- the resulting offspring is Therefore has a different set of alleles from those of either its parents.
So many variables for fertilisation to occur
- which particular egg released
- sperm production
- survival in tract
- which month which egg ect
Explain inheritance of epigenetic factors
Epigenetic Factors are changes in gene expression without the change in DNA
(Acetylation and Methylation)
- some epigenetic tales remain in place as genetic information passes from generation to generation, a process called EPIGENETIC INHERITANCE
Examples of Epigenetic Inheritance
- Women born in the german hunger strike (1944-45) due to WW2 gave birth to smaller babies than average babies
Then in 1960s when those women had kids their babies were also very small. - Fathers who have been smoking before puberty had sons which were at higher risks of obesity and other mental health problems than sons of fathers who did not smoke.
What can affect epigenetic inheritance?
- Smoking
- Malnutrition
- Baby lotions containing peanut oil leading to rise of peanut allergies
- High levels of anxiety in women lead to children with risk of asthma
- Excessively clean children leads to skin conditions such as eczema
Explain Mutations
Mutations are a change in gene or chromosome leading to a new characteristic in an organism
May result in a protein not being made or a faulty version being produced. In some cases, an essential enzyme impaired
Occurs during DNA replication - copying of code - affects phenotype of organism
Causes: chemicals, radiation, UV, X-rays, radioactive waste, nuclear explosions
What is a species
A species is a group of organisms that, having many characteristics in common, are able to interbreed and produce fertile offspring under natural conditions.
