3.7.1 Inheritance

• define genotype the genetic constitution of an organism (the alleles it has for a gene)
• define phenotype the expression of the genes and its interaction with the environment
• define homozygous  a pair of homologous chromosomes carrying the same alleles for a single gene
• define heterozygous  a pair of homologous chromosomes carrying 2 different alleles for a single gene
• define recessive allele an allele only expressed if no dominant allele is present
• define dominant allele an allele that will always be expressed in the phenotype
• define codominant both alleles are equally dominant and expressed in the phenotype
• define multiple alleles more than 2 alleles for a single gene
• define sex-linkage a gene whose locus is on the X chromosome
• define autosomal linkage  genes that are located on the same chromosome (not the sex-chromosomes)
• define epistasis  when one gene modifies or masks the expression of a different gene at a different locus
• define monohybrid  genetic inheritance cross of a characteristic determined by one gene
• define dihybrid  genetic inheritance cross for a characteristic determined by two genes
• outline the steps you would do when using genetic diagrams to work out probability for offspring  1. state parental genotypes2. work out gametes by putting in punnet square3. fill in genotypes and phenotypes
• define gene  a sequence of bases on a DNA molecule that codes for a protein (polypeptide)which results in a characteristic
• define locus the fixed position of a gene on a chromosome
• alleles of a gene are found at the same locus on each chromosome in a pair
• define homozygote  an organism that carries two copies of the same allele
• define heterozygote  an organism that carries two different alleles
• define carrier  a person carrying an allele which is not expressed in the phenotype but that can be passed on to offspring
• humans are diploid organisms (we have two sets of chromosomes) so we have two alleles for each gene gametes contain only one allele for each gene when gametes from two offspring fuse together, the alleles they contain form the genotype of the offspring producedat each locus, the genotype can be homozygous or heterozygous  
• what are genetic diagrams used for  to predict the genotypes and phenotypes of the offspring produced if 2 parents are crossed
• what is monohybrid inheritance  the inheritance of a characteristic controlled by a single gene
• what do monohybrid crosses show  the likelihood of the different allelels of a gene being inherited by offspring of certain parents
• what is an example of codominance in humans  allele for sickle-cell anaemia
• what can you use a dihybrid cross to do  to look at how 2 different genes are inherited at the same time
• what is the phenotypic ratio  the ratio of different phenotypes in offspring
• genetic diagrams allow you to predict the phenotypic ratios in F1 and F2 offspring
• what is F2 second set of offspring
• when might you not get the expected phenotypic ratio  because of sex linkage autosomal linkage or epistasis
• the genetic information for gender is carried on two sex chromosomes 
• when is a characteristic sex-linked? when the allele that codes for it is located on a sex chromosome
• where are most genes on the sex chromosomes carriedwhy only on the x chromosome (x-linked genes)because y chromosome is smaller than x chromosome and carries fewer genes
• males are more likely than females to show recessive phenotypes for genes that are sex linkedbecause males only have 1 x chromosome so only have 1 allele for sex linked genes - they express the chracterisitc of this one allele even if its recessive
• what are x-linked disorders genetic disroders caused by faily alleles on sex chromosomes (x chromosome)e.g. colour blindness and haemophilia
• what is an autosome  a chromosome that isnt a sex chromosome
• what are autosomal genes the genes located on autosomes
• genes on the same autosome are linked
• what are genes on the same autosome linkedwhen wont this happen because they'll stay together during independent segregation of chromosomes in meiosis 1 and their alleles will be passed on to offspring together wont happen if crossing over splits them up first
• the closer together two genes are on the autosome, the more closely linked they are because crossing over is less likely to split them up