genetics and inheritance
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genetics and inheritance



genetics and inheritance

genetics and inheritance



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genetics and inheritance Presentation Transcript

  • 1. UNIT 4: GENETICS AND INHERITANCE Campbell & Reece: Chapters 14 and 15
  • 2. 1. WHAT IS GENETICS • Genetics: The study of heredity. • Heredity is the relations between successive generations. • Why do children look a little bit like their parents but also different? • What is responsible for these similarities and differences?
  • 3. 2. MENDEL’S GENETICS • Gregory Mendel is the father of Genetics. • Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments. • Advantages of pea plants for genetic study: Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another.
  • 4. • Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another.
  • 5. • He also used varieties that were true-breeding (organisms with only one variety of a type e.g. red flowers can only produce red flowers) • In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization • The true-breeding parents are the P generation. • The hybrid offspring of the P generation are called the F1 generation • When F1 individuals self-pollinate, the F2 generation is produced
  • 6. • When Mendel crossed contrasting, true- breeding white and purple flowered pea plants, all of the F1 hybrids were purple
  • 7. • When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white • Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation.
  • 8. • Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids. • Mendel called the purple flower color a dominant trait and the white flower color a recessive trait • What Mendel called a “heritable factor” is what we now call a gene • He did 7 other crosses using different traits and found the same phenomenon.
  • 9. • Mendel noted that the gene for flower color for example exists in two versions, one for purple flowers and the other for white flowers • These alternative versions of a gene are now called alleles • Each gene is found at a specific locus (position) on a specific chromosome.
  • 10. • The two alleles at a locus on a homologous chromosome pair may be identical, as in the true-breeding plants – they are then said to be homozygous for that trait/gene. • Alternatively, the two alleles at a locus may differ – they are said to be heterozygous for that gene/trait. • If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance (we refer to it as its phenotype), and the other (the recessive allele) has no noticeable effect on
  • 11. • Mendel then formulated the law of segregation, states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes • Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism.
  • 12. • An organism traits are indicated via its genotype and phenotype.  Genotype: The genetic composition of the gene, indicated by letters e.g. GG, Gg, gg. (A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele)  Phenotype: The external appearance of the gene e.g. Brown hair, white hair.
  • 14. TWO TYPES OF GENETIC CROSSES • MONOHYBRID CROSSES: A cross between 2 organisms where we only look an one pair of contrasting traits. • DIHYBRID CROSS: A cross between 2 organisms where we look at two pairs of contrasting traits at the same time.
  • 15. MONOHYBRID CROSS -EXAMPLE • Determine the outcome/ F1 generation of a cross between a homozygous tall plant and a homozygous short plant. Tall plants are dominant over short plants.
  • 16. STEPS TO SOLVE A CROSS PROBLEM 1. What trait are we looking at? 2. Choose a letter to represent the trait. 3. See if you can identify which trait is dominant – allocate the capital letter to that trait. 4. Identify the recessive trait and allocate a lower case letter to that trait. 5. Determine the genotypes of the parents. – Homozygous dominant – Two capital letters e.g. GG Homozygous recessive – Two lower case letter. E.g. gg Heterozygous – One capital letter and one lower case letter e.g. Gg 1. Start with cross
  • 17. SOLUTION 1. Trait – Size of plant. 2. Letter chosen to represent size of plant = T/t 3. Tall plants are dominant. (Given in problem) – Given the – “T” (capital T) 4. Short plants are recessive – given the “t” (lower case t) 5. One parent is homozygous tall – TT other parent is homozygous short - tt
  • 18. CROSS SHOWN AS A GENETIC DIAGRAM Why? Tall is dominant over short plants – Babies have both alleles: tall and short
  • 19. Cross shown as a punnet square
  • 21. MONOHYBRID CROSS –EXAMPLE 2 A heterozygous blue eyed rabbit is crossed with a rabbit with pink eyes. What is the possibility of the babies being born with pink eyes?
  • 22. SOLUTION 1. Trait: eye colour of rabbit. 2. Letter used: E/e 3. Dominant trait: Blue eyes (Why? The first rabbit is heterozygous – both alleles – but blue is being expressed in rabbit eyes.) = E 4. Recessive trait: pink eyes = e 5. Rabbit one – heterozygous: Ee Rabbit two – homozygous: ee (why?) The only way that a rabbit can have pink eyes expressed externally is if both alleles code for pink eyes.
  • 23. EXAMPLE OF A DIHYBRID CROSS Determine the F2 generation of a cross between yellow round seeded peas and wrinkled green seeded peas. Yellow and round seeds are dominant.
  • 24. • Using a dihybrid cross, Mendel developed the law of independent assortment • The law of independent assortment states that each pair of alleles segregates independently from another pair of alleles during gamete formation. • Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes • Genes located near each other on the same chromosome tend to be inherited together.
  • 25. Dihybrid cross • In humans there is a disease called Phenylketonuria (PKU) which is caused by a recessive allele. People with this allele have a defective enzyme and cannot break down the amino acid phenylalanine. This disease can result in mental retardation or death. Let “E” represent the normal enzyme. Also in humans in a condition called galactose intolerance or galactosemia, which is also caused by a recessive allele. Let “G” represent the normal allele for galactose digestion. In both diseases, normal dominates over recessive. • If two adults were heterozygous for both traits, what are the chances of having a child that is completely normal? • Has just PKU? • Has just galactosemia? • Has both diseases?
  • 26. EG Eg eG eg EG EEGG EEGg EeGG EeGg Eg EEGg EEgg EeGg Eegg eG EeGG EeGg eeGG eeGg eg EeGg Eegg eeGg eegg P1 EeGg x EeGg Meiosis F1 EG Eg eG eg EG Eg eG eg
  • 27. 4. DEGREES OF DOMINANCE •Complete dominance One allele suppresses the expression of the other allele. • Incomplete dominance: phenotype of F1 hybrids is somewhere between the phenotypes of the 2 parental varieties – neither allele completely dominant (White x Red = Pink) • Codominance, 2 dominant alleles affect the phenotype in separate, distinguishable ways. (Red and white flowers = White and red visible.)
  • 28. 5. MULTIPLE ALLELES  Most genes exist in populations in more than two allelic forms.  For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.  The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither.
  • 29. 6. PLEIOTROPY  Most genes have multiple phenotypic effects, a property called pleiotropy  For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
  • 30. 7. Polygenic Inheritance  Polygenic inheritance is an additive effect of two or more genes on a single phenotype  Skin color in humans is an example of polygenic inheritance.
  • 31. 8. DETERMINING THE SEX OF A BABY  In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome  Only the ends of the Y chromosome have regions that are homologous with the X chromosome  The SRY gene on the Y chromosome codes for the development of testes.
  • 32. X and Y CHROMOSOMES
  • 33.  Females are XX, and males are XY  Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome.
  • 35. 9. Inheritance of Sex-Linked Genes  The sex chromosomes have genes for many characters unrelated to sex  A gene located on either sex chromosome is called a sex-linked gene  In humans, sex-linked refers to a gene on the larger X chromosome.  For a recessive sex-linked trait to be expressed  A female needs two copies of the allele  A male needs only one copy of the allele.  Sex-linked recessive disorders are much more common in males than in females.
  • 36. Some disorders caused by recessive alleles on the X chromosome in humans: • Color blindness • Duchenne muscular dystrophy • Hemophilia