6.1.2 Patterns Of Inheritance Flashcards
Discontinuous variation
Discontinuous variation is genetic variation producing discrete (discontinuous)
phenotypes, so two or more non-overlapping categories. These traits tend to be
monogenic, so different alleles have very different effects on the phenotype. Can be
represented using a bar chart. Is unaffected by the environment.
Continuous variation
Continuous variation is genetic variation that produces phenotypic variation where
quantitative traits vary by very small amounts between one group and the next. Such
characteristics tend to be polygenic (so each of the alleles involved has an additive
effect).The greater the number of gene loci involved, the more continuous the variation
(the greater the range). Such data can be represented on a histogram. It is influenced by
the environment.
How does meiosis create genetic variation ?
Crossing over of chromatids where pairs of homologous chromosomes line up and
exchange some of their genetic material
Independent assortment of chromosomes – there are various combinations of
chromosome arrangement
Allele
A version of a gene
Locus
the specific position of a gene on a chromosome, the two alleles of a gene are
found at the same loci on the chromosome pairs
Phenotype
visible characteristics of an organism
Genotype
genetic makeup of an organism
Codominance
both alleles contribute to the phenotype
Dominant
only a single allele is required for the characteristic to be expressed, that
allele is always expressed in the phenotype
Reccesive
the characteristic is only expressed if there is no dominant allele present
Homozygous
two identical alleles
Heterozygous
two different alleles
Discuss mendel’s work
Mendel studied pea plants and found through his crossings that the tall allele is dominant
and the dwarf allele is recessive, the purple flower is dominant and the white flower is
recessive, the axial flower position is dominant and the terminal position is recessive, the
full pod shape is dominant and then constricted pod shape is recessive, the yellow seed
colour is dominant and the green seed colour is recessive, and the round seed shape is
dominant and the wrinkled seed shape is recessive. All of these characteristics that he
studied were monogenic. He was able to determine this by getting true breeding strains
for each trait (where the trait was shown to be unchanged in the strain over
generations), and by keeping accurate and quantitative records of the data he obtained,
which he then analysed.
What is monogenic inheritance, give an example
when a phenotype or trait is controlled by a single gene. For instance, cystic fibrosis where the individuals with doubly recessive genotype are affected. When parental (P1) organisms, one homozygous recessive and one homozygous dominant, are crossed, all offspring are heterozygous for the trait. When these offspring the F1 (F for filial) generation interbreed, ¾ of the offspring (F2 generation) will show the dominant phenotype,and ¼ will show the recessive phenotype. Huntington’s is an example of a monogenic condition in humans. It is dominant, but a double dose of the allele is fatal, so if both parents have it then 50% of the offspring will have the condition, 25% will be unaffected, and 25% will not survive to be born.
Describe what a dihybrid cross is
inheritance of two genes simultaneously. The two
genes are inherited independently of each other and so each gamete has
one allele for each of the two gene loci. This means that during fertilisation
any one of an allele pair can combine with any one of another allele pair.
The probability of any two traits being inherited together if they are not
linked is the product of the individual probabilities
Define linkage
is the phenomenon where genes for different characteristics are located at
different loci on the same chromosome and so are inherited together.
Describe sex linkage and give example
Sex linkage – expression of an allele dependent on the gender of the individual as the
gene is located on a sex chromosome. Sex Chromosomes (XY in men, XX in women)
are not fully homologous, but a small part on one matches up with a small part on the other
so that they can pair up before meiosis. Many genes on the X chromosome are not to do
with sex determination (i.e: involved in metabolic functions) and have no partner allele on
the Y chromosome. So if a female inherits an X chromosome with a faulty allele she will
likely have a functioning allele for the same gene on the other X chromosome, whereas if
a man has a faulty allele on his X chromosome then he will suffer from a genetic disease
as he will not have a functioning allele for that gene. So women can be hetero or
homozygous for X-linked genes and men can’t be as they only have one, so they are
functionally haploid, or hemizygous, for X-linked genes. Women are homogametic as
they have two identical sex chromosomes, but in birds, butterflies and moths males are
homogametic and females are heterogametic. Examples of recessive sex linked
disorders are hemophilia (X^H is dominant for normal blood clotting, X^h is recessive and
leads to abnormal blood clotting)and colourblindness.
Describe autosomal linkage
genes which are located on the same chromosome (which is not
a sex chromosome) and tend to be inherited together in the offspring. This occurs as the
chromosome, not the gene, is the unit of transmission during meiosis, so linked genes are very unlikely to be separated by independent assortment. If linked genes are not affected
by crossing over of non-sister chromatids during prophase 1 then they are always inherited
as one unit.
Describe epistasis
The interaction between non-linked gene loci where one masks expression
of the other. The genes can be located at different loci on the same (or different) genes or
chromosomes. They interact to form one phenotypic characteristic, and can work
antagonistically or in a complementary fashion. As the loci are not linked , they assort
independently during gamete formation. Epistasis reduces the number of phenotypes
produced in the F2 generation of dihybrid crosses and therefore reduces phenotypic
variation.
What is the chi-squared test
The chi squared test is a statistical test which can be used to establish whether the
difference between observed and expected results is small enough to occur purely due to
chance.
When can we use use chi-squared test
● It can be used if the sample size is sufficiently large, that is over 20. It can only be
used for discontinuous variation (categoric) data in the form of raw counts, and if
there are no zero scores in the raw data. We should also have a strong biological theory
to use to predict expected values.
● The chi squared test can be used to determine whether the null hypothesis is correct or
not. The null hypothesis is the assumption that there is no significant difference
between observed and expected results.
● The value obtained is compared to the critical value, and in a case where the value
obtained is less than the critical value, the null hypothesis is accepted as the difference
due to chance is not significance
● Whereas in a case where the x2 value is greater than critical value, the null hypothesis is
rejected meaning that the difference between observed and expected results is not due
to chance, as it is significant.
How to calculate chi-squared
To calculate Chi squared:
1. Minus the expected from the observed of each value, squaring the result and then
dividing it by the expected.
2. Add all the values you gained from step 1 together to get x squared
3. Determine the degrees of freedom (this is the number of categories -1)
4. Determine the value of p from a distribution table
5. Decide whether the difference is significant at the p=0.05 level
What is the Hardy weinburg equation
The Hardy-Weinberg Equation can be used to estimate the frequency of alleles in a population and to see whether a change in allele frequency is occurring in a population over time. ● p = the frequency of the dominant allele (represented by A) q = the frequency of the recessive allele (represented by a) For a population in genetic equilibrium: ● p + q = 1.0 (The sum of the frequencies of both alleles is 100%.) ● (p + q)2 = 1 so p2+ 2pq + q2 = 1 ● The three terms of this binomial expansion indicate the frequencies of the three genotypes: ● p2 = frequency of AA (homozygous dominant) ● 2pq = frequency of Aa (heterozygous) ● q2 = frequency of aa (homozygous recessive)
What are the hardy-weinburg principles assume
★ The population is large enough to make sampling error negligible
★ Mating within the population occurs at random
★ There is no selective advantage for any genotype, so no selection
★ There is no mutation, migration, or genetic drift
