Chapter 7

1. Chapter 7 Heredity

2. Key knowledge • Transmission of heritable characteristics • Genes as units of inheritance • Eukaryote chromosomes, alleles, prokaryote chromosomes, plasmids • Cell reproduction: cell cycle, DNA replication, apoptosis, binary fission, gamete production, inputs and outputs of meiosis • Variation: genotype, phenotype, continuous variation, discontinuous variation • Patterns of inheritance • One gene locus – monohybrid cross, including dominance, recessiveness, codominance, multiple alleles. • Two gene loci – dihybrid cross • Pedigree analysis – autosomal, sex-linked inheritance, test cross

3. Heredity • Heredity: is the study of inheritance. Principles of heredity and patterns of inheritance were first established by an Austraian monk, Gregor Mendel. • Genetics: study of the mechanism and patterns of inheritance through the transmission of coded chemical instructions from one generation to the next. • Genes: segments of DNA that directed the formation of particular structural and functional protein of cells. • Genes may code for more than one kind of protein • Genome – the sum aof all the DNA in the cell of an organism

4. Reproduction – transmission of heritable traits

5. Reproduction – transmission of heritable traits • Cells pass on instructions for growth and development from one generation to the next during mitosis and cytokinesis. • Living things that originate from one parent are said to reproduce asexually. They usually resemble the parent because they only have one source of hereditary information. • Organisms that reproduce sexually have two sources of hereditary material, which are carried in specialised reproductive cells called gametes.

6. Chromosomes and Heredity • Genetic information carried in its DNA molecules. Eukaryotes in the nucleus • Prokaryotes – DNA lies free within the cell.

7. Chromosomes and heredity – Chromosomes of eukaryotes • Chromatin – DNA & protein • Nuclear division – DNA molecules appear as double structures each coiled around histone proteins linked at the centromere. • Chromosomes are normally visible only during cell division. Eukaryotes chromosomes exist in pairs.

8. Changing shapes of chromosomes • Metacentric • Centromere in the centre • Submetacentric • Centromere nearer one end than the other • Acrocentric • Centromere close to the end • Telocentric • Centromere on the end

9. Chromosomes and heredity – Karyotype • Karyotype – standard form used to display and analyse chromosomes. • Humans – somatic or body cell – 46 chromosomes which form 23 pairs of which 22 are matched or homologous. • One chromosome of each pair comes from the male parent via the sperm cell and the other from the female parent via the egg cell (ovum) • Matched pairs autosomes • 23rd pair is matched in females (XX) but unmatched in males (XY) is called a heterosome (hetero-different) • Sex chromosomes

10. Chromosomes and heredity – Karyotype • Diploid – number of chromosomes in each body (somatic) cell – 2n • Trait – characteristic • Locus – the position a gene occupies in a chromosome. • Allele – alternative form of a gene

11. Barr Bodies • In female mammals one of two X chromosomes is inactivated, or turned off and condenses.

12. Cell division

13. Cell cycle • Mitosis – process of nuclear division in somatic cells and cytokinesis, the division of the cell, results in the formation of two diploid daughter cells which contain identical sets of chromosomes. • Meiosis –nuclear division results in the formation of four daughter cells, which each contain half the number of chromosomes of the original nucleus – haploid.

14. Cell Cycle

15. DNA replication • DNA replication (S Phase) occurs between 2 phases of growth (G1 and G2). • G1 phase – cells make biochemicals and organelles • S phases begins with enzyme DNA helicase unzipping the helix of double stranded DNA exposing nucleotide bases. – happens along a small section at a time • Hydrogen bonds hold two strands of DNA together are weak and the enzyme is easily able to separate them.

16. DNA Replication • Junction between the unwound single strands of DNA and the intact double helix is called the replication fork. • Fork moves along the parental DNA strand so that there is a continuous unwinding of the parental strand. • Free nucleotides attach to the exposed bases according to the base-pairing rule with the help of DNA polymerase. • DNA ligase seals the new short stretches of nucleotides into a continuous strand that rewinds.

18. DNA ReplicationSemi-conservative replication • Outcome of DNA replication is two double-helix DNA molecules, each consisting of one parental strand and one new strand. • One of the two strand is conserved from one generation to the next, while the other strand is new.

19. Nuclear division in Somatic Cells - Mitosis • Interphase • Chromosomes not visible and cannot be clearly distinguished. • Immediately before mitosis centrioles visible, chromatin threads become shorter and thicker – visible under light microscope.

20. Mitosis • Four main stages of mitosis • Prophase • During prophase, chromatin threads condenses and sister chromatids become visible – held togheter by a centromere. A spindle forms and the nucleolus disappears from view. The nuclear membrane break downs. • Metaphase • Chromosomes move to the centre of the cell and line up along the equator. • Anaphase • Chromatids separate and move to opposite poles of the spindle. They are chromosomes. • Telophase • Chromosomes lengthen and become less visible. A new nuclear envelope forms and nucleoli reform.

21. Mitosis • Cytokinesis • Following mitosis, cytokinesis occurs. • Cytoplasm of plant cells divided with formation of a cell plate which eventually becomes the cell wall. • Animal cells do not have a cell wall. Cytoplasm divides by a a process called cleavage. Cleavage furrow.

23. Apoptosis • Programmed cell death • Webbing in between fingers. • Crucial part of the development process • Apoptosis plays a large role in cell cycle. • Keeps tight rein on cell division.

24. Division of Prokaryotic Cells • Prokaryotic bacterial cells simply replicate their single DNA strand. • Following replication and separation, a wall forms across the cell and divides into two cells. • Binary fission.

25. Nuclear division in sex cells Meiosis

26. Meiosis

27. Stages of Meiosis

30. Stages of Meiosis – Meiosis I • Prophase I • Chromosomes condense, nucleolus disappeares, spindle forms. Homologous chromosomes side by side – synapsis. Homologous chromosomes may coil around each other. Later they may move apart by the chromatids remain in contact at points called chiasmata. • Metaphase I • Nuclear envelope breaks down and the homologous chromosomes move together to the equator of the spindle. • Anaphase I • Homologous chromosomes move towards the opposite poles of the spindle. Disjunction of pairs of homologous chromosomes is independent of other chromosomes. • Telophase I • Cell starts to divide across its middle and nuclear envelopes form around the two nuclei. The spindle breaks down. • Interphase • Brief interphase usually occurs. DNA does not duplicate during this interphase

31. Stages of Meiosis – Meiosis II • Prophase II • New spindle forms at right angles to the first • Metaphase II • Chromosomes move to the equator of the spindle • Anaphase II • Chromatids separate and move apart from each other. Chromatids become the chromosomes of daughter cells. When they reach the poles, the cells enter • Telophase II • Spindle disappears, chromosomes regain their thread-like form and new nuclear envelopes and nucleoli form. • 4 haploid cells (n)

33. Increasing variation • Mitosis – offspring have same DNA as parent cell. • Spontaneous or induced changes in DNA  mutations can result in variation of characteristics. • Environmental conditions stable – little variation for species survival, asexual reproduction is ideal – less energy needed for complex and specialised reproductive processes. • Sexual reproduction can result in an increase in variation between generations as there is input from two parents. • If environmental conditions change, some members of a species may have slight variations in characteristics, which may or may not give them a competitive edge in survival.

34. Patterns of Inheritance

35. Monohybrid inheritance • Gene – a segment of DNA that controls a particular trait • Allele – An alternative form of a gene. For example in pea plants there is a gene which controls pod colours. It has two alleles A – green pods, a – yellow pods • Dominant – refers to a trait which is expressed when the organism has either one or two copies of the allele present for that particular trait. • Phenotype – the characteristics shown by a particular organism. The phenotype is a result of an organism’s genotype and any influences due to the environment. • The genotypes, AA and Aa, would be expected to result in pea pods showing a green phenotype. The genotype aa, would result in pea pods with a yellow phenotype.

36. Monohybrid inheritance • Monohybrid involves one gene, with two different alleles. • One trait is dominant and the other is recessive. It is important to note that the traits are dominant or recessive not the actually alleles. • Pure breeding parents – homozygous for that gene • Individuals with two different alleles = hybrids – heterozygous for that gene.

37. Monohybrid inheritance • Complete dominance

38. The law of segregation • Inherited characteristics are controlled by genes that occur in pairs. • The alleles of each gene will separate during meiosis so that each gamete only have one allele of each gene

39. The law of independent assortment • Each gene pair is inherited independent of other gene pairs during gamete formation. • Alleles of one gene assort independently of the alleles of another gene during meiosis.

40. Identifying recessive alleles • If the parents do not express the recessive trait, they must be homozygous. In such a case. • The first appearance of the recessive trait within a family is usually in the F1 generation. • 25% of this F1 will express the trait • Both males and females can express the trait unless it is a recessive sex-linked gene.

41. If one parent does express the recessive trait • That parent must be homozygous • The trait will not be expressed in every generation • Both males and females can express the trait unless it is a recessive sex-linked gene

42. Identifying dominant traits • If the trait is dominant, at least some offspring in all generations will show the trait • The trait is passed from the affected parent to at least 50% of the F1 generation. If at least one parent is homozygous for the dominant trait, then all offspring will show it. If both parents are heterozygous and the other is heterozygous for the recessive trait then 50% of offspring will show it. • Any parents that does not express the trait does not transmit it to any of the F1. • Both males and females can express and transmit the trait.

43. Test crosses • Are a way of determining whether an organism is homozygous or heterozygous • Test crosses are used to determine an organism’s genotype. • We test cross with individuals that are know to be homozygous recessive. • By examining the offspring we are able to identify the genotype of the individual.

44. Test crosses • Can be used to determine an unknown genotype. • In a test cross an individual showing the dominant phenotype but of unknown genotype is crossed with a homozygous recessive genotype. • Consider the following examples of crosses between Aa X aa, and AA x aa

45. Test crosses • If the unknown genotype is Aa, then at least half of the offspring could be expected to show the recessive character. Genotype ratio Aa:aa = 1:1. • If unknown genotype is AA, then none of the offspring would show the recessive character. All of the offspring would show the dominant feature. Genotype: all Aa.

46. Dihybrid inheritance

47. Relationships and Dominance

48. Co-dominance and incomplete (partial) dominance

49. Co-dominance and incomplete (partial) dominance • Co-dominance refers to a pattern of inheritance in which heterozygous individuals have a phenotype different form the phenotypes of both of the homologous individuals, and the effects of the two different alleles are both evident in this phenotype. • Inheritance of ABO blood types in humans is an example of this. • One gene locus controls the production of certain antigens present on the surface of red blood cells. This gene has three alleles.

50. Blood gene alleles • Notice that when dealing with co-dominant of incompletely dominant inheritance, alleles are shown as capital letters with a superscript/subscript letter or number distinguishing the alleles.