Inheritance Flashcards
INHERITANCE
what is a homologous chromosome?
A pair of chromosomes of the smae length and centromere position for the genes corresponding to the same loci.
what is a phenotype?
the appearance of an organism due to expression of it’s genes. can be affected by the environment
what is a genotype?
genetic make-up of an organism
what is a gene?
a length of DNA coding (on a chromosome) for a particular protein
What is a loci?
the allele the gene is found at a fixed position is called the locus. (the position of a gene on a chromosome)
what is an allele?
one form of a gene
what is meant by the term recessive?
an allele that is only expressed if there are no dominant alleles present
what is meant by the term dominant?
an allele that is always expressed when present
where are chromosomes inherited from?
one from each parent, so we have a maternal and paternal strand.
what is autosomal dominant? draw an example using pedigree diagram
cannot be recessive as two affected parents could not have unaffected offspring. therefore parents must be heterozygous
what is autosomal recessive? draw a diagram showing this?
cannot be dominant as two unaffected parents could not have an affected offspring. therefore parents must be heterozygous
what are diploid organisms?
have two alleles for the each gene
what are haploid organisms?
gametes (sex-cells) contain only one allele for each gene
Monohybrid Inheritance
What is Monohybrid inheritance?
is the inheritance of a single gene
what do Monohybrid crosses show?
show the likelihood of different alleles of that gene being inherited by offspring of certain parents
what does a monohybrid cross look like?
parents genotypes, then gametes then the possible genotypes of offsprings then the phenotype
In mice, the gene for eye colour has two alleles. the alle for black eyes is dominant, while the alleles for red eyes is recessive. Choose a suitable symbols for these alleles, then draw a punnet square to show the probable results of a cross between a heterozygous black-eyes mouse and a red-eyed mouse.
F1 generation - 50:50 ratio
Co-dominance and Multiple Alleles
What is codominance?
alleles have equal dominance, and so they are both expressed. the traits appear together in the phenotype of hybrid organisms e.g blood groups
What is incomplete dominance?
we get a blending of the traits so that the third phenotype is something in the middle (red x white = pink)
Draw a diagram to show blood group phenotypes and genotypes. also practice question.
what are multiple alleles?
when there are more than two alleles for the same gene
Give an example?
eye-colour and blood groups
Sex-linked inheritance
what genes are said to be sex-linked?
Genes located on the sex chromosomes
how is sex determined?
Sex in mammals is determined by two chromosomes, known as sex chromosomes or heterosomes.
what are autosomes?
non- sex chromosomes)
which sex chromosome is larger?
The X chromosome is larger and contains about 2000 genes, whereas the Y chromosome is smaller and contains fewer than 100.
what type of conditions are sex-linked/ x-linked?
Haemophilia
Duchenne muscular dystrophy
Red–green colour blindness
what type of conditions are Y-linked?
It is argued that there is little room on the Y chromosome for anything other than genes controlling testes formation and function.
why is X-linkage more common?
the X chromosome is larger and part of it does not have a homologous section on the Y chromosome, therefore only one allele of a gene will be present and so will always
be expressed.
what are homologous regions?
do not carry sex-determining genes
what are non-homologous regions on a chromosome?
carry sex-determining genes and other genes
When does the son inherit a sex-linked disease?
A son ALWAYS gets his X chromosome from his mother!
when does the daughter inherit a sex-linked disease?
Daughter’s ALWAYS get the father’s X chromosome and therefore the diseased allele if her father has it.
what is meant by X-linked dominant? draw a diagram to show this
Sex linkage can not be confirmed. 100% incidence of affected daughters from an affected father suggests x-linked dominance.
what is meant by X-linked recessive? draw a diagram to show this
sex linkage cannot be confirmed. 100% incidence of affected sons from an affected mother suggests X-linked recessive
Dihybrid crosses
what do Dihybrid crosses show?
Dihybrid crosses are those where we consider the inheritance of two
characteristics at the same time.
each characteristic is determined by..?
a different gene, potentially located
on a different chromosome.
give an example?
Freckles and widows peak
Who was Gregor Mendel?
know as the father of modern genetics. In the 1950’S AND 1960’S he established the foundations of inheritance - dominant and recessive characteristics. He had no idea about DNA, genes but knew how they were inherited
How did he prove his findings on inheritance?
he used peas and studies them by crossing them together to see what offsprings they made.
Cross a Purebreed tall (TT), purebreed yellow seed (YY) plant with a purebreed short (tt) purebreed green seed (yy) plant.
draw diagram, all offspeings should be heterozygous TtYy
what is this generation called?
F1 generation
if the heterozygous offsprings were to be crossed over and produce offsprings, they wold be called the ..?
F2 generation
What did Mendel’s independent assortment principle state?
The homologous chromosomes line up randomly on the centre line in Metaphase I so the combination of alleles in the gametes is RANDOM
Draw a genetic diagram of the heterozygous offsprings being crossed (f2 generation)
what ratio do these two heterozygous parents produce in offsprings
9:3:3:1 always
How did mendel figure out which phenotypes were dominant or recessive?
From his monohybrid trials Mendel discovered:
- allele for yellow colour was dominant to green
- allele for round peas was dominant to wrinkled peas.
Autosomal Linkage
what is autosomal linkage?
when two genes are located on the same chromosomes and this means that the alleles for each gene will be inherited together
Who found out the existence of Autosomal linkage?
Bateson, Saunders and Punnet 1905
how did they do this?
they studied two genes:
- flower colour (P, purple and p, red)
- shape of pollen grains (L, long, l, round)
and crossed PPLL and ppll
F1 generation: All offspring were purple and long (PpLl) As expected.
However F2 generation gave suprising results
Why were they supprised by the results of the F2 generation?
Gave a 3:1 ratio (just like a monohybrid cross)
Explain why there isn’t a perfect 3:1 ratio?
The parental combinations of alleles (purple long and red round) seem to be inherited as almost a 3:1 ratio
As though they were behaving as a single character
These genes are called LINKED GENES (Autosomal linkage)
i.e. The genes are transmitted together (on the same chromosome).
Why are there different combination of alleles? (4 to be exact)
what do we call these combinations?
These combinations have come about because of CROSSING OVER between the linked alleles on their chromosomes during Meiosis 1. These are called RECOMBINANTS
explain what happens during crossing over?
Homologous chromosomes are aligned. Chromatids cross over forming bivalents and chiasmata. chromatids break off and exchange genetic material. so we are left with different combination of alleles.
How to test if inheritance is significant?
always chi squared test
Epistasis
what is epistasis?
Epistasis is a form of a gene interaction in which one group makes a phenothopic expression of another
what are epistatic alleles?
the alleles that are masking the effect. (of dominant or recessive traits)
what are hypostatic alleles?
the alleles whose effect is being masked.
how can epistatic problems be solved?
Dihybrid crosses
give an example of epistasis?
e.g colour of labradors
gene 1 : controls colour represented by B
- labrador has a dominant B = black for BB/Bb
- Two bb (recessive) = brown for bb
gene 2: controls the expression of gene and is represented by E/e
what does gene 1 represent?
the phenotype /genotype
what does gene 2 represent?
the hypostatic allele represented by E.
E = no effect e = has effect of masking
so the labradors can either be Black, brown and Golden. what are all the possible genotypes for all phenotypes.
Add in epistatsis dominant,
FINISH
