B17 inheritance

• genotype is the genetic constitution of an organism, it describes all the alleles that an organism has
• phenotype is how the genetic constitution is expressed, it is the observable/biochemical characteristics that an organism has
• a gene is a length of DNA nucleotide bases that codes for a specific polypeptide
• an allele is one of the different forms of a gene
• in homologous chromosomes, if the allele on each chromosome is the same the organism is homozygous, and if the allele on each chromosome is different the organism is heterozygous
• the three types of allele are dominant, recessive, codominant
• dominant alleles are always expressed if present
• recessive alleles are only expressed if no dominant alleles are present
• codominant alleles are both expressed at once
• monohybrid inheritance is when a single characteristic passes on from one generation to the next
• in genetic crosses, use the symbols provided to represent alleles/genes, but if you have to choose then use the first letter unless that is a duplicate, and choose letters which are different in uppercase and lowercase
• in genetic crosses, uppercase is for the dominant alleles, and lowercase is for the recessive alleles
• in genetic crosses, always clearly label the parents, state the gametes produced, clearly label and circle the gametes
• in genetic crosses, use a punnett square to show the results of random crossing of gametes
• in genetic crosses, after the punnett square, state the phenotypes of each genotype, and the numbers of each type, and always write the dominant allele first
• ratios produced by genetic crosses will not be perfectly achieved due to the random nature of fertilisation
• be careful to write a ratio the correct way round, as shown in the question
• dihybrid inheritance is when two characters, determined by two different genes located on different chromosomes, are passed from one generation to the next
• the theoretical ratio for a heterozygous cross involving dihybrid inheritance is 9:3:3:1
• the law of independent assortment states that each member of a pair of alleles may combine randomly with another
• chromosomes arrange themselves at random on the equator during meiosis, so any one of the alleles of one gene can combine with any one of the alleles of the other gene
• fertilisation is random, so any one of the four gametes of one organism can combine with any one of the four gametes of the other organism
• for codominant alleles, or multiple alleles, use the same method with a punnett square, but remember that if the offspring has a mixture of two equally dominant alleles then it has a mixture of phenotypes
• multiple alleles is when there are more than two alleles for a characteristic
• in humans the sex chromosomes are X and Y, females have XX and males have XY, so females produce identical gametes but males produce two different gametes
• a gene is sex-linked if it is carried on the X or Y chromosome
• the X chromosome is much longer than the Y chromosome, so for most of the X chromosome there is no equivalent homologous portion on the Y chromosome
• X-linked genetic disorders are inherited because males can only get their X chromosome from their mother, and if this has the disorder, they will be affected, even if she was a carrier
• in pedigree charts, males are represented by squares and females are represented by circles
• in pedigree charts, if a shape is shaded, the individual has that characteristic
• any two genes that occur on the same chromosome are linked
• X and Y are the sex chromosomes, any of the others are called autosomes
• autosomal linkage is when two or more genes are carried on the same autosome
• in autosomal linkage, if there was no crossing over, all linked genes would stay together during meiosis, and would therefore pass into offspring together
• in autosomal linkage, only two gametes can be produced, although four are possible, but the question has to tell you which they are
• epistasis arises when the allele of one gene affects or masks the expression of another in the phenotype of the organism, e.g. one controls the options but another controls whether the chosen option is expressed
• the theoretical ratio of a heterozygous cross involving epistasis is 9:4:3
• the $chi^2$test is used to test the null hypothesis of an investigation, it can be used if the sample size is greater than 20, the data is discrete, the data is in the form of a raw count, and you want to compare experimental results with theoretical results
• in the $chi^2$test formula, $\chi^2$represents chi squared, $\Sigma$represents the sum of, O represents the observed numbers, and E represents the expected numbers
• once a value of $\chi^2$is calculated, you need to read it off of a chi squared distribution table, which requires you to look along the row with the correct degrees of freedom, read it until you find where the calculated value would fit, then read down that column to find the critical value