5 Genes and inheritance Flashcards
What did Mendel find that for every feature or character he investigated?
- A ‘heritable unit’ (what we now call a gene) is passed from one generation
to the next. - The heritable unit (gene) can have alternative forms (we now call these
different forms alleles). - Each individual must have two alternative forms (alleles) per feature.
- The gametes only have one of the alternative forms (allele) per feature.
- One allele can be dominant over the other.
What did Mendel notice about pea plants?
Mendel noticed that many of the features of pea plants had two alternative forms.
What is an example of these alternative forms?
For example, plants were either tall or very short (called a ‘dwarf’ variety); they either had purple or white flowers; they produced yellow seeds or green seeds.
Were there intermediate forms?
There were no intermediate forms.
What do you mean by intermediate forms?
There were no pale purple flowers or
green/yellow seeds or intermediate height plants.
What is an image which shows some features of pea plants used by Mendel in his breeding experiments?
What did Mendel decide to do?
Mendel decided to investigate, systematically, the results of cross breeding plants that had contrasting features.
What were these plants known as?
These were the ‘parent plants’.
What were these parent plants referred to in genetic diagrams?
P.
What did Mendel do to each plant?
He transferred pollen from one experimental plant to another.
What did Mendel make sure about each plant?
He also made sure that the plants could not be self-fertilised.
What did he do in terms of seeds?
He collected all the seeds formed, grew them and noted the features that each
plant developed.
What were these plants called that grew from the seeds?
These plants were the first generation of offspring, called the F1 generation.
What was the F1 generation?
Offspring formed from breeding the
parent organisms.
What did he do to these plants?
He did not cross-pollinate these plants, but allowed them to self-fertilise.
What is cross-pollination?
Transfer of pollen from an anther of one plant to a stigma of a different plant of the same species.
What is self-fertilise?
Fertilization that occurs when male and female gametes produced by the same organism unite.
After the F1 generation self-fertilised, what did he do?
Again, he collected the seeds, grew them and noted the features that each plant developed.
What were these new plants called?
These plants formed the second generation of offspring or F2 generation.
What is the F2 generation?
The offspring formed from breeding
individuals from the F1 generation.
What is an image that shows what happened when mendel used pure-breeding tall and pure-breeding dwarf plants as his parent plants?
Were these results specific to only the pea plants?
Mendel obtained similar results when he carried out breeding experiments
using plants with other pairs of contrasting characters.
What are two particular things that Mendel noticed in all his experiments?
- All the plants of the F1 generation were of one type. This type was not a
blend of the two parental features, but one or the other. For example, when
tall and dwarf parents were crossed, all the F1 plants were tall. - There was always a 3:1 ratio of types in the F2 generation. Three-quarters
of the plants in the F2 generation were of the type that appeared in the F1
generation. One-quarter showed the other parental feature. For example,
when tall F1 plants were crossed, three-quarters of the F2 plants were tall
and one-quarter were dwarf.
How did Mendel use his findings?
Mendel was able to use his findings to work out how features were inherited,
despite having no knowledge of chromosomes, genes or meiosis.
What can we do with his results nowadays?
Nowadays we can use our understanding of these ideas to explain Mendel’s results.
What are seven explanations of Mendel’s results?
- Each feature is controlled by a gene, which is found on a chromosome.
- There are two copies of each chromosome and each gene in all body cells, except the gametes.
- The gametes have only one copy of each chromosome and each gene (i.e.
one allele). - There are two alleles of each gene.
- One allele is dominant over the other allele, which is recessive.
- When two different alleles (one dominant and one recessive) are in the same cell, only the dominant allele is expressed (shown in the appearance of the organism).
- An individual can have two dominant alleles, two recessive alleles or a
dominant allele and a recessive allele in each cell
What is an allele?
Different forms of a gene.
What does dominant mean?
Allele of a gene that is expressed in
the heterozygote.
What does recessive mean?
Allele that is not expressed in the phenotype when a dominant allele of the gene is present (i.e. in the
heterozygote).
What can we use as an example to explain Mendel’s results?
We can use the cross between tall and dwarf pea plants as an example.
What are the two different alleles in pea plants?
In pea plants, there are tall and dwarf alleles of the gene for height.
What is the symbol that we will use for the tall allele?
T
Why have we used a capital T?
This is because tall is the dominant allele.
What is the symbol that we will use for the short allele?
t
What does genotype mean?
It is the alleles an organism has for a certain characteristic.
Why have we used a lowercase t?
This is because short i the recessive allele.
What does phenotype mean?
How a gene is expressed. The ‘appearance’ of an organism resulting from its genotype.
What is the sequence followed when writing genetic diagrams?
- Phenotype of parents.
- Genotype of parents.
- Gametes (sex cells).
- Punnet square.
- Ratio.
- Percentage.
What is an image that shows the results of crosses using tree-breeding tall and dwarf pea plants?
What is important to note about the ratios given in genetic crosses?
It is important to remember that in genetic crosses, ratios such as 3:1 are
predicted ratios.
Are the actual numbers likely to fit the predicted ones?
In breeding experiments the actual numbers of offspring are unlikely to exactly fit a 3:1 ratio.
What is an example of how the ratios are solely just predicted?
For example, one of Mendel’s experiments produced 787 tall plants and 277 dwarf plants. This is a ratio of 2.84:1, not quite the expected 3:1.
So will the real ratios usually be around the same as predicted ratios?
Yeah, but not exactly.
What is the reason for getting another ratio?
The reason for this is that there are a number of factors that affect survival of the plants.
What are some of the factors that effect the survival of the plants?
- Some pollen may not fertilise some ova.
- Some seedlings may die before they
mature.
What are these events called?
These are unpredictable or ‘chance’ events.
Why do we still continue to use these predictions then?
The numbers that Mendel found were statistically close enough to the expected 3:1 ratio, and he found the same thing when he repeated his experiments with other
characteristics
What can you not tell solely by just looking at pea plants?
Whether it is homozygous, or heterozygous.
What does homozygous mean?
Genotype with the same alleles of a gene, e.g. AA or aa.
What does heterozygous mean?
Genotype with different alleles of a gene, e.g. Aa.
How would both of these genotypes appear?
Equally tall because the tall allele is dominant.
What information would be useful to find out the genotype of the plant then?
It would help if you knew the genotypes of its parents.
What could we then do with the genotypes of its parents?
You could then write out
a genetic cross and perhaps work out the genotype of your tall plant.
What can you do if tou don’t know the genotypes of the parents?
The only way you can find out is by
carrying out a breeding experiment called a test cross.
What is a test cross?
Cross of an organism showing the dominant phenotype with one showing the recessive phenotype. The F1 from the cross shows whether the parent is homozygous dominant or heterozygous.
What is the factor under investigation in a test cross?
In a test cross, the factor under investigation is the unknown genotype of an organism showing the dominant phenotype.
What genotype could a tall pea plant have?
TT or Tt.
What must you control?
You must control every other possible variable including the genotype of the plant you breed it with.
What is the only genotype you can be certain of?
It is the genotype of plants showing the recessive phenotype (in this case dwarf plants).
What must be the genotype of these plants which show the recessive phenotype?
They must have the genotype ttt.
What do we do in an example?
You must breed the ‘unknown’ tall pea plant (TT or Tt) with a dwarf pea plant (tt).
How can we predict the outcomes for each?
You can write out a genetic cross for both possibilities (TT ́ tt and Tt ́ tt) and predict the outcome for each.
What do you then do with the results?
You can then compare the result of the breeding experiment with the predicted outcome, to see which result matches the prediction most closely.
What is an example of this test cross?
What would we expect to get in terms of offspring from this test cross?
- All the offspring to be tall if the tall parent was homozygous (TT).
- Half the offspring to be tall and half to be dwarf if the tall parent was
heterozygous (Tt).
Why are genetic crosses useful?
Writing out a genetic cross is a useful way of showing how genes are passed
through one or two generations, starting from the parents.
How can we show the family history of a genetic condition?
We can use a diagram called a pedigree.
What is a pedigree?
Diagram showing a family tree for an inherited characteristic.
What is polydactyly?
It is an inherited condition in which a person develops an extra digit
(finger or toe) on the hands and feet.
What kind of allele is polydactyly determined by?
It is determined by a dominant allele.
What does the recessive allele do?
The recessive allele causes the normal number of digits to develop.
What symbol would we use for the polydactyly allele?
D.
What symbol would we use for the normal-number allele?
d.
What are the possible genotypes and phenotypes when using these symbols?
- DD.
- Dd.
- dd.
What does DD mean in this context?
Person has polydactyly (has two dominant polydactyly alleles).
What does Dd mean in this context?
Person has polydactyly (has a dominant polydactyly allele and a recessive normal allele).
What does dd mean in this context?
Person has the normal number of digits (has two recessive, normal-
number alleles).
What is a diagram of a pedigree showing the inheritance of polydactyly in a family?
What is the information that we can extract, just based on this pedigree?
- There are four generations shown (individuals are arranged in four horizontal lines).
- Individuals 4, 5 and 6 are children of individuals 1 and 2 (a family line
connects each one directly to 1 and 2) - Individual 4 is the first-born child of 1 and 2 (the first-born child is shown to
the left, then second born to the right of this, then the third born and so on) - Individuals 3 and 7 are not children of 1 and 2 (no family line connects them
directly to 1 and 2). - Individuals 3 and 4 are father and mother of the same children – as are 1
and 2, 6 and 7, 8 and 9, 12 and 13, 14 and 15 (a horizontal line joins them).
What can we work out from a pedigree in terms of alleles?
It is usually possible to work out which allele is dominant from a pedigree.
How do you find out which allele is dominant?
You look for a situation where two parents show the same feature and at least one child shows the contrasting feature.
What is an example of this in this specific pedigree?
Individuals 1 and 2 both have
polydactyly, but children 4 and 6 do not.
How can we explain why children 4 and 6 do not have polydactyly?
- The normal alleles in 4 and 6 can only have come from their parents (1 and 2),
so 1 and 2 must both carry normal alleles.
-1 and 2 show polydactyly, so they must have polydactyly alleles as well. - If they have both polydactyly alleles and normal alleles but show
polydactyly, the polydactyly allele must be the dominant allele.
What can we know now that which allele is dominant?
We can work out most of the genotypes in the pedigree.
What must the people with the normal number of digits have as their genotype?
All the people with the normal number of digits must have the genotype dd (if they had even one D allele, they would show polydactyly).
What must all the people with polydactyly have in terms of their alleles?
All the people with polydactyly must have at least one polydactyly
allele (they must be either DD or Dd).
From here, how could we work out the genotypes of people with polydactyly?
To do this we need to remember that people with the normal number of digits must inherit one ‘normal-number’ allele from each parent, and also that people with the normal number of digits will pass on one ‘normal- number’ allele to each of their children.
What can we say about any person with polydactyly who has children with the normal number of digits?
They must be heterozygous.
Why must they be heterozygous?
Because the child must have
inherited one of their two ‘normal-number’ alleles from this parent.
Who else would be heterozygous in this pedigree?
That any person with polydactyly who has one parent with the normal number of digits must also be heterozygous (the ‘normal-number’ parent can only have passed on a ‘normal-number’ allele).
What is an image of a pedigree showing the inheritance of polydactyly in a family, with details of genotypes added?
Which individuals are we still uncertain about?
We are still uncertain about individuals 5, 8 and 12.
What could individuals 5, 8, and 12 be?
They could be homozygous or heterozygous.
What is some information that we need to bear in mind about individuals 1 and 2 to find out about 5?
They are both heterozygous.
What is a diagram of possible outcomes from a genetic cross between two parents, both heterozygous for polydactyly?
What is an explanation of this diagram?
Individual 5 could be any of the outcomes indicated by the shading. It is impossible to distinguish between DD and Dd.
What is our sex determined by?
Our sex – whether we are male or female – is determined by the X and Y chromosomes.
What are the sex chromosomes?
The pair of chromosomes that determine sex in humans. XX in females, XY in males.
What are these X and Y chromosomes?
The sex chromosomes.
As well as the 44 non-sex chromosomes what are two chromosomes in all cells of females (apart from egg cells)?
XX.
As well as the 44 non-sex chromosomes what are two chromosomes in all cells of males (apart from sperm cells)?
XY.
What is our sex effectively determined by?
The presence or absence of the Y chromosome.
What is a diagram showing the determination of sex in humans?
What is an explanation of this diagram?
In any one family, however, this ratio may well not be met.
Why may this ratio not be met?
Predicted genetic ratios are
usually only met when large numbers are involved.
What is the overall ratio of male and female births in all countries?
It is 1:1.
What have all of the genetic crosses so far had in common?
All of the genetic crosses that you have seen in this chapter have been examples of inheritance involving single genes.
Why have they involved single genes?
The reason for this is that it is easier to
draw genetics diagrams and explain what is happening if we start by considering alleles of a single gene.
What do these single-gene diagrams disregard?
The fact that many characteristics are controlled by two or more genes working together.
What is this called?
Polygenic inheritance.
What is polygenic inheritance?
Characteristics controlled by two or more genes working together
What is a good example of polygenic inheritance?
A good example is human skin colour.
What does darker skin contain?
Darker skins contain greater amounts of a black pigment called melanin. This is controlled by several genes, which act together to determine the amount of melanin in the skin.
What does each gene have in terms of alleles and melanin production?
Each gene has alleles that promote melanin production and alleles which do not.
What do these genes allow?
This produces a wide range of phenotypes.
What are other human characteristics determined by several genes?
- Human height.
- Body mass (weight).
What are polygenes?
It is the name given to a group of several genes working together to determine a characteristic, e.g. height
in humans.
What are most phenotypic features a result of?
Polygenic inheritance rather than single genes.
