Mendel ( ) Mendel studied inheritance in peas. His idea was that organisms passed on separate characteristics via “inherited factors” (we now call genes). He recognised that some “inherited factors” were dominant, whilst others were recessive. The importance of Mendel’s work was not recognised until after his death because: 1.DNA, genes and chromosomes had not been discovered. (Chromosomes were first seen under a microscope in about The idea of genes being small sections of chromosomes which could be inherited then followed). 2. People struggled to understand his theories. Genetic crosses key terms (HT only) Homozygous = When an organism has two identical copies of the same allele. For example TT or tt. Heterozygous =When an organism has two different alleles of the same gene. For example Tt. Phenotype = The appearance or characteristics caused by a particular allele. Genotype = The combination of alleles found in the cell. For example TT, Tt or tt. Family trees: These can be used to track the inheritance of genetic diseases over several generations. If the disease shown is caused by a dominant allele, what are the genotypes of parents 1 and 2 (use the letters A and a for alleles)? State clearly if they are homozygous or heterozygous. 1 = Aa, heterozygous. 2 = aa, homozygous. B2 Genetics: chromosomes; genes; DNA; recessive; heterozygous; death; phenotype; homozygous; genotype; allele; Parents

Genetic disorders Genetic disorders can be inherited. They are caused by faulty alleles. The presence of disease causing alleles can be confirmed by embryo screening. Parents can then make informed decisions about termination of a pregnancy. Polydactyly This is caused by a dominant allele so can be passed on by only one parent who has the disorder. Sufferers have extra fingers or toes. Show the outcome of a cross between two parents, one of which is healthy, whilst the other has one copy of the disease causing allele. Let d = healthy allele, D = polydactyly allele Parents phenotype: healthy x polydactyly Parents genotype: dd x Dd Offspring: 1 healthy : 1 polydactyly ½ (50%) chance of healthy baby, ½ (50%) chance of baby with polydactyly. Cystic fibrosis This is caused by a recessive allele, so sufferers need to inherit two copies, one from each parent. The parents may be suffers of the disease or symptomless carriers. The disorder affects the cell membranes, especially in the lungs, causing a build up of sticky mucus. Show the outcome of a cross between two parents, both of which are healthy, but carriers of cystic fibrosis. Let F = healthy allele, f = cystic fibrosis allele Parents phenotype: healthy x healthy (carrier) (carrier) Parents genotype: Ff x Ff Offspring: 3 healthy : 1 cystic fibrosis ¾ (75%) chance of healthy baby, ¼ (25%) chance of baby with cystic fibrosis. B2 Genetic disorders: carriers, lungs, recessive, two, dominant, screening, mucus, phenotype, faulty, fingers, polydactyly, termination, one, toes, healthy, (letters and outcomes of crosses not included) gametesFf FFF (healthy)Ff (carrier) f ff (cystic fibrosis gametesdd DDd (polydactyly) ddd (healthy)

Genetic fingerprinting: Each person (except identical twins) has unique DNA. Genetic fingerprinting is used to identify people. Related people share some bands eg the child and Man B share bands , so Man B is likely to be the child’s father. Sex chromosomes In human body cells one of the 23 pairs of chromosomes carries the genes that determine sex. In females the sex chromosomes are the same (XX), in males they are different (XY). Parents phenotype: male x female Parents genotype: XY x XX Offspring: Ratio 1 male : 1 female ½ (50%) chance of male baby, ½ (50%) chance of female baby. Genetic terms Gene = small section of DNA. Alleles = different forms of the same gene. Dominant = an allele that controls a characteristic even if present on only one chromosome. Recessive = an allele that controls a characteristic only if the dominant allele is absent. Genetic code (higher tier) Each gene codes for a particular combination of amino acids which makes a specific protein. B2 Genetics terms, sex chromosomes, genetic code and genetic fingerprinting: related, 12, XX, protein, gene, father, fingerprinting, dominant, alleles, recessive, XY, females, unique, 16, 23, amino, sex, males. (No outcome values listed). gametesXX X XX (female) YXY (male)