inheritance Flashcards
what is a diploid cell?
A cell that contains 2 complete sets of chromosomes (2n)
These chromosomes contain the DNA necessary for protein synthesis and cell function
Nearly all cells in the human body are diploid with 23 pairs (46) of chromosomes in their nucleus
what is a haploid cell?
a cell that contains one complete set of chromosomes (n)
In other words they have half the number of chromosomes compared to diploid cells
Humans have haploid cells that contain 23 chromosomes in their nucleus
These haploid cells are called gametes and they are involved in sexual reproduction
For humans they are the female egg and the male sperm
Haploidy and diploidy are terms that can be applied to cells across different species
They describe the number of sets of chromosomes, not the total number of chromosomes
whate is a gamete?
reproductive (sex) cells that fuse with another during fertilisation
zygote
the diploid cell from which a cell develops
- forms from the fusion of gametes.
What happens during fertilization ?
- the nuclei of gametes fuse together to form the nucleus of the zygote
Both gametes must contain the correct number of chromosomes in order for the zygote to be viable. If a zygote has too many or too few chromosomes it may not survive
For a diploid zygote this means that the gametes must be haploid
n + n = 2n
what does meiosis produce?
haploid gametes during sexual reproduction
What is the first cell division of meiosis called?
a reduction division
This is a nuclear division that reduces the chromosome number of a cell
In humans the chromosome number is reduced from 46 (diploid) to 23 (haploid)
The reduction in chromosome number during meiosis ensures the gametes formed are haploid
chromosomes have a characteristic shape
They have a fixed length
The position of the centromere is in a particular location
These characteristic features allow for each chromosome to be identified in a photomicrograph
In photomicrographs chromosomes are often grouped into their homologous pairs
homologous chromosomes
Carry the same genes in the same positions
Are the same shape
During fertilization what type of zygote forms?
a diploid zygote is formed
In a zygote one chromosome of each homologous pair comes from the female gamete and the other comes from the male gamete
Having the same genes in the same order helps homologous chromosomes line up alongside each other during meiosis
Although homologous pairs of chromosomes contain the same genes in the same order they don’t ….
necessarily carry the same alleles (form) of each gene
meiosis
is a form of nuclear division that results in the production of haploid cells from diploid cells
It produces gametes in plants and animals that are used in sexual reproduction
what are the 2 divisions of meiosis called:
meiosis I and meiosis II
Within each division there are the following stages: prophase, metaphase, anaphase and telophase
prophase I
DNA condenses and becomes visible as chromosomes
DNA replication has already occurred so each chromosome consists of two sister chromatids joined together by a centromere
The chromosomes are arranged side by side in homologous pairs
A pair of homologous chromosomes is called a bivalent
As the homologous chromosomes are very close together the crossing over of non-sister chromatids may occur. The point at which the crossing over occurs is called the chiasma (chiasmata; plural)
In this stage centrioles migrate to opposite poles and the spindle is formed
The nuclear envelope breaks down and the nucleolus disintegrates
Metaphase I
The bivalents line up along the equator of the spindle, with the spindle fibres attached to the centromeres
Anaphase I
The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to opposite ends of the spindle
The centromeres do not divide
Telophase I
The chromosomes arrive at opposite poles
Spindle fibres start to break down
Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I
Cytokinesis
This is when the division of the cytoplasm occurs
Cell organelles also get distributed between the two developing cells
In animal cells: the cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell which contracts, dividing the cytoplasm in half
In plant cells, vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The vesicles merge with each other to form the new cell surface membrane and also secrete a layer of calcium pectate which becomes the middle lamella. Layers of cellulose are laid upon the middle lamella to form the primary and secondary walls of the cell
The end product of cytokinesis in meiosis I: two haploid cells
These cells are haploid as they contain half the number of centromeres
Meiosis II
There is no interphase between meiosis I and meiosis II so the DNA is not replicated
The second division of meiosis is almost identical to the stages of mitosis
Prophase II
The nuclear envelope breaks down and chromosomes condense
A spindle forms at a right angle to the old one
Metaphase II
Chromosomes line up in a single file along the equator of the spindle
Anaphase II
Centromeres divide and individual chromatids are pulled to opposite poles
This creates four groups of chromosomes that have half the number of chromosomes compared to the original parent cell
Telophase II
Nuclear membranes form around each group of chromosomes
Cytokinesis in Meiosis II
Cytoplasm divides as new cell surface membranes are formed creating four haploid cells
The cells still contain the same number of centromeres as they did at the start of meiosis I but they now only have half the number of chromosomes (previously chromatids)
Meiosis I or Meiosis II
Homologous chromosomes pair up side by side in meiosis I only
This means if there are pairs of chromosomes in a diagram or photomicrograph meiosis I must be occurring
The number of cells forming can help distinguish between meiosis I and II
If there are two new cells forming it is meiosis I but if there are four new cells forming it is meiosis II
The distinguishing features at each stage of Meiosis I
Prophase I: Homologous pairs of chromosomes are visible
Metaphase I: Homologous pairs are lined up side by side along the equator of spindle
Anaphase I: Whole chromosomes are being pulled to opposite poles with centromeres intact
Telophase I: There are 2 groups of condensed chromosomes around which nuclei membranes are forming
Cytokinesis: Cytoplasm is dividing and cell membrane is pinching inwards to form two cells
The distinguishing features at each stage of Meiosis II
Prophase II: Single whole chromosomes are visible
Metaphase II: Single whole chromosomes are lined up along the equator of the spindle in single file (at 90 degree angle to the old spindle)
Anaphase II: Centromeres divide and chromatids are being pulled to opposite poles
Telophase II: Nuclei are forming around the 4 groups of condensed chromosomes
Cytokinesis: Cytoplasm is dividing and four haploid cells are forming
Meiosis - sources of Genetic variation
2 ways that increase the genetic diversity of gametes produced
crossing over and independent assortment (random orientation) result in different combinations of alleles in gametes
Meiosis - sources of Genetic variation
Crossing over
is the process by which non-sister chromatids exchange alleles
Process:
During meiosis I homologous chromosomes pair up and are in very close proximity to each other
The non-sister chromatids can cross over and get entangled
These crossing points are called chiasmata
The entanglement places stress on the DNA molecules
As a result of this a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes
There is usually at least one, if not more, chiasmata present in each bivalent during meiosis
Crossing over is more likely to occur further down the chromosome away from the centromere
Meiosis - sources of Genetic variation
Independent assortment
is the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I
The different combinations of chromosomes in daughter cells increases genetic variation between gametes
In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the equator of the spindle
Each pair can be arranged with either chromosome on top, this is completely random
The orientation of one homologous pair is independent / unaffected by the orientation of any other pair
The homologous chromosomes are then separated and pulled apart to different poles
The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up
To work out the number of different possible chromosome combinations the formula 2n can be used, where n corresponds to the number of chromosomes in a haploid cell
For humans this is 223 which calculates as 8 324 608 different combinations
How does Meiosis create genetic variation between the gametes produced ?
by an individual through crossing over and independent assortment
This means each gamete carries substantially different alleles
During fertilization any male gamete can fuse with any female gamete to form a zygote
This random fusion of gametes at fertilization creates genetic variation between zygotes as each will have a unique combination of alleles
There is an almost zero chance of individual organisms resulting from successive sexual reproduction being genetically identical
what does each chromosome consist of?
a long DNA molecule which codes for several different proteins
what is a gene?
A length of DNA that codes for a single polypeptide or protein
define locus (plural: loci)
The position of a gene on a chromosome
define alleles
is a different form of the same gene
Each gene can exist in two or more different forms
Different alleles of a gene have slightly different nucleotide sequences but they still occupy the same position (locus) on the chromosome
Example of alleles
One of the genes for coat colour in horses is Agouti
This gene for coat colour is found on the same position on the same chromosome for all horses
Hypothetically there are two different forms (alleles) of that gene found in horses: A and a
Each allele can produce a different coat colour:
Allele A → black coat
Allele a → chestnut coat
define genotype of an organism
the alleles possessed by an organism
The genotype of an individual affects their phenotype
define homozygous
When the two allele copies are identical in an individual
define heterozygous
When the two allele copies are different in an individual
define phenotype
is the observable characteristics of an organism
A horse that has two black coat alleles A has the genotype AA and is homozygous. The phenotype of this horse would be a black coat
In contrast a horse that has one black coat allele A and one chestnut coat allele a would have the genotype Aa and is heterozygous
define dominant
Some alleles are dominant: they are always expressed in the phenotype
This means they are expressed in both heterozygous and homozygous individuals
define recessive
they are only expressed in the phenotype if no dominant allele is present
This means that it is only expressed when present in a homozygous individual
If for horses the allele A for a black coat is dominant and the allele a for a chestnut coat is recessive the following genotypes and phenotypes occur:
Genotype AA → black coat
Genotype Aa → black coat
Genotype aa → chestnut coat
define codominance
Sometimes both alleles can be expressed in the phenotype at the same time
When an individual is heterozygous they will express both alleles in their phenotype
When writing the genotype for codominance the gene is symbolised as the capital letter and the alleles are represented by different superscript letters, for example I^A
^ : to the power of
A good example of codominance can be seen in human blood types
The gene for blood types is represented in the genotype by I and the three alleles for human blood types are represented by A, B and O
Allele A results in blood type A (IAIA or IAIO) and allele B results in blood type B (IBIB or IBIO)
If both allele A and allele B are present in a heterozygous individual they will have blood type AB (IAIB)
Blood type O (IOIO) is recessive to both group A and group B alleles
When a homozygous dominant individual is crossed with a homozygous recessive individual what are the offspring called ?
the F1 generation
All of the F1 generation are heterozygous
If two individuals from the F1 generation are then crossed, the offspring they produce would be called?
the F2 generation
Why is a test cross used and what does it deduce?
the genotype of an unknown individual that is expressing a dominant phenotype
The individual in question is crossed with an individual that is expressing the recessive phenotype
The resulting phenotypes of the offspring provide sufficient information to suggest the genotype of the unknown individual
If there are any offspring expressing the recessive phenotype then the unknown individual must have a heterozygous genotype
there are 2 types of linkages in genetics. What are they?
sex linkage and autosomal linkage
linkage
Sex linkage
There are two sex chromosomes: X and Y
Women have two copies of the X chromosome (XX) whereas men have one X chromosome and one shorter Y chromosome (XY)
Some genes are found on a region of a sex chromosome that is not present on the other sex chromosome
As the inheritance of these genes is dependent on the sex of the individual they are called sex-linked genes
Most often sex-linked genes are found on the longer X chromsome
Haemophilia is well known example of a sex-linked disease
Sex-linked genes are represented in the genotype by writing the alleles as superscript next to the sex chromosome. For example a particular gene that is found only on the X chromosome has two alleles G and g. The genotype of a heterozygous female would be written as XGXg. A males genotype would be written as XGY
linkage
Autosomal linkage
This occurs on the autosomes (any chromosome that isn’t a sex chromosome)
Two or more genes on the same chromosome do not assort independently during meiosis
These genes are linked and they stay together in the original parental combination
What does monohybrid inheritance look at?
how the alleles for a single gene are passed on from one generation to the next
Known information about the genotypes, phenotypes and the process of meiosis are used to make predictions about the phenotypes of offspring that would result from specific breeding pairs
When two individuals sexually reproduce there is an equal chance of either allele from their homologous pair making it into their gametes and subsequently the nucleus of the zygote
This means there is an equal chance of the zygote inheriting either allele from their parent
Genetic diagrams are often used to present this information in a clear and precise manner so that predictions can be made
These diagrams are called?
Punnett squares
The predicted genotypes that genetic diagrams produce are all based on chance
There is no way to predict which gametes will fuse so sometimes the observed or real-life results can differ from the predictions
look at worked examples save my exams 16.2.2 and 16.2.3
What do dihybrid crosses look at?
how the alleles of two genes transfer across generations
When caan a test cross be used?
to deduce the genotype of an unknown individual that is expressing a dominant phenotype
The individual in question is crossed with an individual that is expressing the recessive phenotype. This is because an individual with a recessive phenotype has a known genotype
The resulting phenotypes of the offspring provides sufficient information to suggest the genotype of the unknown individual
Results for test cross for monohybrid test
If no offspring exhibit the recessive phenotype then the unknown genotype is homozygous dominant
If at least one of the offspring exhibit the recessive phenotype then the unknown genotype is heterozygous
Results for test cross - dihybrid
If no offspring exhibit the recessive phenotype for either gene then the unknown genotype is homozygous dominant for both genes
If at least one of the offspring exhibit the recessive phenotype for one gene but not the other, then the unknown genotype is heterozygous for one gene and homozygous dominant for the other
If at least one of the offspring exhibit the recessive phenotype for both genes then the unknown genotype is heterozygous for both genes
look at worked examples 16.2.4 save my exams
chi squared
16.2.5 save my exams
what can a genotype effect?
the phenotype of an organism
A gene codes for a single protein
The protein affects the phenotype through a particular mechanism
what can the phenotype of an individual also be affected by?
the environment
TYR gene & albinism
Humans with albinism lack the pigment melanin in their skin, hair and eyes
- causes them to have very pale skin, very pale hair and pale blue or pink irises in the eyes
There is a metabolic pathway for producing melanin:
1.The amino acid tyrosine is converted to DOPA by the enzyme tyrosinase
2.DOPA is converted to dopaquinone again by the enzyme tyrosinase
3.Dopaquinone is converted to melanin
tyrosine → DOPA → dopaquinone → melanin
A gene called TYR located on chromosome 11 codes for the enzyme tyrosinase
There is a recessive allele for the gene TYR that causes a lack of enzyme tyrosinase or the presence of inactive tyrosinase
Without the tyrosinase enzyme tyrosine can not be converted into melanin
HBB gene & sickle cell anaemia
Sickle cell anaemia is a condition that causes individuals to have frequent infections, episodes of pain and anaemia
Humans with sickle cell anaemia have abnormal haemoglobin in their red blood cells
β-globin is a polypeptide found in haemoglobin that is coded for by the gene HBB which is found on chromosome 11
There is an abnormal allele for the gene HBB which produces a slightly different amino acid sequence to the normal allele
The change of a single base in the DNA of the abnormal allele results in an amino acid substitution (the base sequence CTC is replaced by CAC)
This change in amino acid sequence (the amino acid Glu is replaced with Val) results in an abnormal β-globin polypeptide
The abnormal β-globin in haemoglobin affects the structure and shape of the red blood cells
They are pulled into a half moon shape
They are unable to transport oxygen around the body
They stick to each other and clump together blocking capillaries
A homozygous individual that has two abnormal alleles for the HBB gene produces only sickle cell haemoglobin
They have sickle cell anaemia and suffer from the associated symptoms
A heterozygous individual that has one normal allele and one abnormal allele for the HBB gene will produce some normal haemoglobin and some sickle cell haemoglobin
They are a carrier of the allele
They may have no symptoms
F8 gene & haemophilia
Factor VIII is a coagulating agent that plays an essential role in blood clotting
The gene F8 codes for the Factor VIII protein
There are abnormal alleles of the F8 gene that result in:
Production of abnormal forms of factor VIII
Less production of normal factor VIII
No production of factor VIII
A lack of normal factor VIII prevents normal blood clotting and causes the condition haemophilia
The F8 gene is located on the X chromosome
This means F8 is a sex-linked gene
Haemophilia is a sex-linked condition
If males have an abnormal allele they will have the condition as they have only one copy of the gene
Females can be heterozygous for the F8 gene and not suffer from the condition but act as a carrier
F8 gene & haemophilia
Factor VIII is a coagulating agent that plays an essential role in blood clotting
The gene F8 codes for the Factor VIII protein
There are abnormal alleles of the F8 gene that result in:
Production of abnormal forms of factor VIII
Less production of normal factor VIII
No production of factor VIII
A lack of normal factor VIII prevents normal blood clotting and causes the condition haemophilia
The F8 gene is located on the X chromosome
This means F8 is a sex-linked gene
Haemophilia is a sex-linked condition
If males have an abnormal allele they will have the condition as they have only one copy of the gene
Females can be heterozygous for the F8 gene and not suffer from the condition but act as a carrier
HTT gene & Huntington’s disease
Huntington’s disease is a genetic condition that develops as a person ages
Usually a person with the disease will not show symptoms until they are 30 years old +
An individual with the condition experiences neurological degeneration; they lose their ability to walk, talk and think
The disease is ultimately fatal
It has been found that individuals with Huntington’s disease have abnormal alleles of the HTT gene
The HTT gene codes for the protein huntingtin which is involved in neuronal development
People that have a large number (>40) of repeated CAG triplets present in the nucleotide sequence of their HTT gene suffer from the disease
The abnormal allele is dominant over the normal allele
If an individual has one abnormal allele present they will suffer from the disease
The Role of Gibberellin in Stem Elongation
In some plants species their height is partially controlled by their genes
The Le gene dictates the height of some plants
It has two alleles: Le and le
The dominant allele Le produces tall plants when present
The recessive allele le produces shorter plants when present (in a homozygous individual)
The gene regulates the production of an enzyme that is involved in a pathway that forms active gibberellin GA1
Active gibberellin is a hormone that helps plants grow by stimulating cell division and elongation in the stem
The recessive allele le results in non-functional enzyme
It is only one nucleotide different to the dominant allele
This causes a single amino acid substitution (threonine -> alanine) in the primary structure of the enzyme
This change in primary structure occurs at the active site of the enzyme, making it non-functional
Without this enzyme no active gibberellin is formed and plants are unable to grow tall
Plants that are homozygous for the recessive allele le are dwarves
Some farmers apply active gibberellin to shorter plants to stimulate growth
The Role of Gibberellin in Stem Elongation
In some plants species their height is partially controlled by their genes
The Le gene dictates the height of some plants
It has two alleles: Le and le
The dominant allele Le produces tall plants when present
The recessive allele le produces shorter plants when present (in a homozygous individual)
The gene regulates the production of an enzyme that is involved in a pathway that forms active gibberellin GA1
Active gibberellin is a hormone that helps plants grow by stimulating cell division and elongation in the stem
The recessive allele le results in non-functional enzyme
It is only one nucleotide different to the dominant allele
This causes a single amino acid substitution (threonine -> alanine) in the primary structure of the enzyme
This change in primary structure occurs at the active site of the enzyme, making it non-functional
Without this enzyme no active gibberellin is formed and plants are unable to grow tall
Plants that are homozygous for the recessive allele le are dwarves
Some farmers apply active gibberellin to shorter plants to stimulate growth
When writing gene by hand
Underline gene to represent it in italics
TYR (underline the word)
