Genetics Flashcards
What are chromosomes
Long molecules of DNA wrapped around proteins called histones
What is a chromatin
Chromatin refers to a mixture of DNA and proteins that form the chromosomes found in the cells of humans and other higher organisms.
What happens when there are multiple chromatins wrapped tightly with one another
Forms a chromosome
How can you identify how many chromosomes there are from looking at a diagram
Count the number of centromeres. As such, even a centromere with one chromatid connected to it is considered one chromosome, which is the same number of chromosomes as a centromere with two chromatids attached to it
What is a gene
Sections along DNA that encode for proteins. It is the gene sequence determining the amino acid sequence for the protein it encodes
What does diploid mean
2 copies of each chromosome/chromatids
(this is the double chromatid thing observed)
What does haploid mean
Non homologus chromosome/chromatid (just 1 chromsome/chromatid)
(this is the single chromatid thing observed)
What does homologous mean
pairing at meiosis and having the same structural features and pattern of genes.
I.e. the genes are in the same places
What are autosomes
These are considered chromosomes numbered from 1-22.
These don’t determine our sex. Homologous (each chromosome pair are same in size)
What are sex chromosomes
This is the 23r chromosome. These determine our sex
What is the chromosome for a male
XY
What is the chromosome for a female
XX
What is the trend in size of chromosomes
As they increase in their numbers (i.e. chromosome 1, chromosome 2, chromosome 3, etc.), it typically goes from large to smallest, with smallest at chromosome 22 and largest at chromosome 1
When chromosomes are homologous what do we observe with the genes
The same gene is found on the same location (locus) on the homologous chromosomes
However, the DNA sequence for the same gene on these chromosomes may still have some variations
What is an allele
Different variants/versions/forms of a gene due to their slight differences in DNA sequence.
Thus, several alleles are possible for one gene. FOr example, there are 3 possible alleles for the ABO gene
However, only 2 different alleles are present at the gene’s locus in an individual
What is a genotype
Combination of alleles for a particular gene
What does a homozygous genotype mean
Means the alleles are the same.
I.e. homozygous dominant, homozygous recessive
What does a heterozygous genotype mean
Means the alleles are different.
What does phenotype mean
The observable or measurable characteristic arising from the genotype and its interactions with the environment
I.e. these are thingss we can see with our naked eye our measurable through lenses such as hair colour
What do dominant alleles do
These are able to mask the effect of recessive alleles
Typically characterised by a capital letter
What are some important things to note about dominant alleles
Not about preferential expression
Not necessarily more common in a population
Not necessarily the normal allele
What do recessive alleles do
They have their effect masked by dominant alleles
Typically characterised by lowercase letter
What are some important things to note about recessive alleles
Encoded proteins may still retain some normal function, but at a reduced capacity or completely dysfunctional
Can be more common in some populations
Don’t necessarily give genetic diseases
What is codominance
Occurs when two different alleles and therefore phenotypes are expressed equally.
I.e. AB blood type
What causes dominant phenotypes
Homozygous dominant genotypes
Heterozygous genotypes
What causes recessive phenotypes
Homozygous recessive genotypes only
What chromosomes are able to determine sex
X and Y chromosomes which is seen in the 23rd pair
What does hemizygous mean
A term that describes a person who has only one copy of a gene rather than the usual two copies. Hemizygosity can occur under normal conditions. For example, because most males have one X chromosome and one Y chromosome, they are hemizygous for genes on those chromosomes.
What are the consequences of having hemizygous sex chromosomes for a male
It means that for either the x or y chromosome, any allele would affect the phenotype of the individual (whether it be recessive or dominant). This is because it doesn’t allow the opportunity for anything heterozygous, and as theres only one that one thing will determine the whole phenotype
What are the different sex linked disorders
X - linked dominant
X - linked recessive
Y- linked
Explain X - linked dominant
It suggests the abnormal allele is dominant and lies on an X chromosome
Lets say that the abnormal allele is dominant A, and the normal allele is recessive ‘a’
For the expression of this phenotype, affected females can have a homozygous dominant xA xA, or they could be heterozygous xA xa
For the expression of this phenotype, as males are hemizygous, there is only one expression; xA y (i.e. one abnormal allele is sufficient to cause the disease)
Explain X - linked recessive
It suggests the abnormal allele is recessive and lies on the x chromosome
Lets say the abnormal allele is recessive a. and the normal allele is dominant A
Affected females have a homozygous recessive genotype xa xa
Meanwhile, for males it doesn’t matter if its recessive because they are hemizygous, this showing the genotype; xa y, which would depict the phenotype a anyways.
In this case, heterozygous females are carriers
What does it mean by carriers
I.e. unaffected but carry an affected genotype (for recessive mothers)
What is haemophilia
This is where there is a clotting deficiency, and abnormal alleles code for proteins that cant participate effectively in blood clotting process
Two types; haemophilia A and haemophilia B
This is an x linked recessive disorder
What is haemophilia A
Most common type
Deficiency of clotting factor 8
What is haemophilia B
Defciency of clotting factor 9
Are females often carriers of haemoophillia
Yes, they can have a heterozygous genotype; generally have a healthy phenotype and may have some mild symptoms of haemophilia.
Severe symptoms is rare but possible despite having the heterozygous genotype
How can hemophilia continue to manifest in females
The process of x chromosome inactivation (lyonisation)
Here one of the two X chromosomes in each diploid cell becomes inactivated during early development. Selection is random. This is to prevent females from having twice as many X chromosome gene products as males
On average, 50% of material X chromosome are 50% of paternal X chromosome is inactivated. However, this could be disproportionate
This could mean that there might be skewed X inactivation, where affected X with haemophilia gene is more active than the unaffected X
Punnet squares
Yes
