Mendel & the gene idea Chapter 14
YOU MUST KNOW Terms associated with genetics problems: P, F1, F2, dominant, recessive, homozygous, heterozygous, phenotype, genotype How to derive the proper gametes when working a genetics problem The difference between an allele and a gene How to read a pedigree How to use data sets to determine Mendelian patterns of inheritance
Alternative versions of genes (alleles) account for variations in inherited characteristics among offspring Ex. 2 alleles for flower color – both are the flower color gene, found at the same locus, but one version codes for purple flowers while the other codes for white
For each characteristic, every organism inherits one allele from each parent
If the two alleles are different, then the dominant allele (shown as a capital letter) will be expressed in offspring, and the recessive allele (shown as a lower case letter) will have no noticeable effect on the offspring
Law of Segregation The two alleles for each trait separate during gamete production -If a parent is homozygous (has two of the same allele- PP or pp), all offspring will get that allele, but if the parent is heterozygous (has two different alleles-Pp), each offspring has a 50% chance of getting either allele
Law of Independent Assortment Alleles on different chromosomes separate independently of each other during anaphase I of meiosis
Phenotype – an organism’s physical expression of a trait Genotype – an organism’s genetic combination of alleles
Test Cross – done to determine if a phenotypically dominant organism is homozygous or heterozygous Note: Really only practical in species that produce large numbers of offspring
Monohybrid cross – examine one trait at a time Dihybrid cross – examine two traits together
Laws of Probability Rule of multiplication – used when calculating the probability that two or more independent events will occur together in a specific combination If you cross two individuals that both have genotypes of AaBbCc, what is the probability of an offspring being AaBBcc? ½ x ¼ x ¼ = 1/32
The Rule of Addition – used when calculating the probability that any of two or more mutually exclusive events occur If two parents are crossed that are AaBb, what are the chances of an offspring being either AABB or aabb? 1/16 + 1/16 = 1/8
Codominance Two alleles are both dominant and are both full expressed in the phenotype Note: Use superscripts to denote different alleles
Incomplete Dominance Hybrids are a blend of the two traits
A gene has more than 2 alleles Multiple alleles A gene has more than 2 alleles *Blood types are also an example of codominance
Pleiotropy – one gene has multiple phenotypic effects
Epistasis – a gene at one locus alters the effects of a gene at another locus
Polygenic Inheritance Two or more genes have an additive effect in a single characteristic
Pedigrees Diagram that shows the relationship between parents and offspring across generations
Recessively inherited disorders Cystic fibrosis – defective cell membrane protein for movement of chloride ions leading to high levels of extracellular chloride which causes thickening of mucus which causes organ malfunction and chronic infections https://www.youtube.com/watch?v=DySt5tLi4G8
Tay-Sachs – codes for a dysfunctional enzyme that doesn’t properly breakdown lipids in the brain. Lipids accumulate causing blindness, seizures, loss of brain function, death by 5. https://www.youtube.com/watch?v=o3yQZp5Rs-o
Sickle-cell disease – hemoglobin gene is mutated, resulting in sickle shaped red blood cells
Lethal Dominant Alleles – require only one copy of the gene to be expressed Usually symptoms are not present until later in life (after genes have already been passed on) Huntington’s disease – degenerative disease of the nervous system, onset usually in 40s https://www.youtube.com/watch?v=HBLrY_nXU_U
Prenatal Genetic Testing Amniocentesis – removal of amniotic fluid an generation of a karyotype Chorionic villus sampling – suction of a small sample of portion placenta that contains fetal cells and generation of a karyotype