who established the basic laws of inheritance?
Gregor Mendel was a monk who studied inheritance in the 1800s using pea plants. he went on to establish the basic laws by which characteristics are inherited from one generation to the next.
define genotype
genotype is the genetic constitution of an organism, determined by the combination of alleles that an organism inherits from it’s parents. the genotype refers to all the alleles that an organism has.
define phenotype
phenotype is the expression of the genotype. it is the observable features and can be affected by the environment.
a gene is a ________ __ _____ __ ___ that _____ ___ ___ ___________. genes exist in ___, __ ____ ________ _____ known as _______. the position of a gene on a particular ___ ________ is known as the _____.
- sequence of bases on DNA
- codes for one polypeptide
- two, or more different forms
- alleles
- DNA molecule
- locus
what are alleles?
alleles are alternate versions of the same gene, which are positioned at the same place, or locus, on homologous chromosomes
how many alleles can a cell have?
although many genes have more than one allelic form, any individual cell will only have 2 alleles for each gene, on on each of the homologous chromosomes that carry these alleles.
when is the phenotypic effect of a recessive allele displayed?
the phenotypic effect of a recessive allele is only displayed in its homozygous state
what happens to the phenotype when codominant alleles are present?
when both alleles occur together (codominance), the phenotype may be a blend
what is a pedigree analysis?
when you are asked to work out what genotypes exist in whole families, based on knowledge of patterns of inheritance in a family tree. this is a pedigree analysis and is a common way of investigating human genetics, where planned breeding is not possible.
what is planned breeding?
male and female plants are chosen - usually pure breeding
what is pure breeding?
crossing with another organism who has the same characteristic - all the offspring also have the same characteristic
what are the important factors for choosing suitable organisms when investigating inheritance of specific genes/alleles? why are these important?
- the organism must show some discontinuous variation
- it must reproduce sexually
- it is pure breeding and its mating can be controlled
- it has a short life cycle and produces large numbers of offspring
- it is convenient to handle
what is it called if inheritance of one pair of alleles is studied?
if inheritance of one pair of alleles is studied, it is called a monohybrid cross
what happened in Mendel’s monohybrid cross?
Mendel crossed a homozygous tall plant with a homozygous dwarf plant and produced all tall offspring. this is called the F1 generation. when the F1 generation was crossed amongst themselves, he always got 3/4 tall plants and 1/4 dwarf plants in the next generation (F2 gen) - a ratio of 3:1. this is called the phenotypic ratio.
what did Mendel conclude from the results of his monohybrid cross?
from his results, Mendel concluded that characteristics were passed from one generation to another via gametes, and that parents must possess two units of information for each characteristic. these ‘units’ we now know as alleles.
explain the modern interpretation of Mendel’s results, including a monohybrid cross for the parents and the F1 generation and an explanation of the ratios
- the gene for height has 2 allelic forms, tall and dwarf, the tall allele being dominant to the dwarf allele, which is thus termed recessive.
- each gamete contains one allele for height. each individual plant has 2 alleles for height in each cell.
- dominant alleles are represented by capital letters, recessive alleles are represented by the same letter but lower case. in this case tall = T and dwarf = t.
- drawing of monohybrid cross for the parents
- drawing of monohybrid cross for the F1 generation
- the ratios represent the probability of obtaining each phenotype. thus in F2 there is a 1 in 4 probability of obtaining a dwarf plant. this probability does not change as a result of previous offspring as there is random assortment and fertilisation of the same types of alleles each time.
what is a method that avoids confusion when drawing lines to show monohybrid crosses?
to avoid confusion when drawing lines to show crosses, a punnet square can be used, where male gametes appear on top and female gametes down the side of the square.
If there are two genotypes for plants with the phenotype tall (TT or Tt), how can each of them be described?
TT - homozygous or pure breeding
Tt - heterozygous or hybrid
what is a testcross/backcross?
a testcross/backcross is a way of determining the genotype of an unknown organism. to do this you cross the unknown with a recessive phenotype, as these always have a homozygous genotype. if any resulting offspring are the homozygous recessive phenotype, the unknown genotype must be heterozygous/Tt. if all the offspring have the dominant phenotype, the unknown genotype must be homozygous dominant/TT.
