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 is an example of this test cross?
What is a diagram of a pedigree showing the inheritance of polydactyly in a family?
What is an image of a pedigree showing the inheritance of polydactyly in a family, with details of genotypes added?
What is a diagram of possible outcomes from a genetic cross between two parents, both heterozygous for polydactyly?
What is a diagram showing the determination of sex in humans?
