Heredity, Mendel, Sex Determination and Evolution

What to remember

• Heredity is the passing of characters from parents to offspring through genes; variation is the difference between individuals of the same species.
• Mendel worked on the garden pea and gave the laws of dominance, segregation and independent assortment (monohybrid ratio 3:1, dihybrid ratio 9:3:3:1).
• In humans the father's sperm (X or Y) decides the sex of the child; evolution is explained by natural selection (Darwin) and supported by fossils and homologous organs.

Heredity and variation

Children look like their parents because they inherit genes. A gene is a segment of DNA that controls a character. Genes lie on chromosomes in the nucleus. A human body cell has 46 chromosomes (23 pairs): 22 pairs of autosomes and one pair of sex chromosomes. A gamete (sperm or egg) has 23 chromosomes. When the gametes fuse, the zygote again has 46.

Variation comes from sexual reproduction, from the mixing of genes, and from mutation (a sudden change in a gene or chromosome). Variation is the raw material for evolution. DNA has the shape of a double helix (Watson and Crick). It is made of four bases: adenine pairs with thymine, and guanine pairs with cytosine.

Terms:

• Trait or character: a feature such as height or seed colour.
• Allele: one of the alternate forms of a gene (for example T for tall and t for dwarf).
• Dominant: the trait that shows in the hybrid. Recessive: the trait that is hidden in the hybrid.
• Genotype: the genetic make-up (TT, Tt, tt). Phenotype: the visible feature (tall, dwarf).
• Homozygous: two same alleles (TT or tt). Heterozygous: two different alleles (Tt).

Mendel's work

Gregor Johann Mendel, an Austrian monk, is called the Father of Genetics. He studied the garden pea (*Pisum sativum*) for several years and published in the 1860s. His work was rediscovered in 1900 by de Vries, Correns and Tschermak.

Why pea? It has many clear contrasting traits, a short life cycle, produces many seeds, and its flowers can be self-pollinated or cross-pollinated by hand.

Seven pairs of traits he studied: seed shape (round/wrinkled), seed colour (yellow/green), flower colour (purple/white), pod shape (inflated/constricted), pod colour (green/yellow), flower position (axial/terminal), plant height (tall/dwarf).

Monohybrid cross: a cross using one pair of traits. Pure tall (TT) × pure dwarf (tt) gives F1 all tall (Tt). Selfing F1 gives F2 with tall and dwarf in a 3:1 phenotypic ratio. The genotypic ratio is 1 TT : 2 Tt : 1 tt (1:2:1).

Dihybrid cross: a cross using two pairs of traits, such as round yellow (RRYY) × wrinkled green (rryy). F1 are all round yellow (RrYy). F2 gives four types in the ratio 9 : 3 : 3 : 1 (round yellow, round green, wrinkled yellow, wrinkled green). New combinations (round green and wrinkled yellow) appear.

Test cross: an organism showing the dominant trait is crossed with a homozygous recessive parent to find its genotype. If all offspring show the dominant trait, the parent is homozygous (TT). If half are recessive, the parent is heterozygous (Tt); the ratio is 1:1.

Sex determination in humans

There are 22 pairs of autosomes and one pair of sex chromosomes. Females have XX; males have XY. The egg always carries X. The sperm carries X or Y in equal numbers. If an X-sperm fertilises the egg the child is a girl (XX); if a Y-sperm fertilises it the child is a boy (XY). So the father decides the sex of the child, and the chance of a boy or a girl is 50% each time. A mother is not responsible for the sex of the child.

Other systems: in birds the female has ZW and the male ZZ. In some reptiles (turtles, crocodiles) the temperature at which eggs are kept decides the sex. In honey bees, males (drones) develop from unfertilised eggs.

Genetic disorders

X-linked recessive disorders are more common in males because males have only one X. A carrier mother can pass the gene to her sons.

Origin of life and evolution

Evolution means the slow change in living things over many generations, giving rise to new species. Life on earth is thought to have begun in the sea. Oparin and Haldane suggested that simple organic molecules formed from non-living matter on early earth. Stanley Miller and Harold Urey (1953) passed electric sparks through a mixture of methane, ammonia, hydrogen and water vapour and obtained amino acids.

Lamarck proposed the theory of use and disuse and the inheritance of acquired characters (the giraffe stretched its neck and its young had longer necks). It is not accepted, because acquired traits do not change the genes. Weismann cut off the tails of mice for many generations and the young still had tails.

Darwin (voyage of HMS Beagle, Galapagos finches) gave the theory of natural selection in *The Origin of Species* (1859). Main ideas: living things produce more young than can survive; there is variation; there is a struggle for existence; those best suited survive (survival of the fittest) and pass on the useful variations. Alfred Russel Wallace reached similar ideas. Hugo de Vries proposed the mutation theory: sudden large changes can form new species.

Example of natural selection: peppered moths in England. When trees became dark with soot, dark moths were less seen by birds and increased in number (industrial melanism).

Evidence for evolution

The age of fossils is estimated by radioactive carbon dating. Fossils in the deeper rock layers are older. Human evolution passed through ancestors such as *Australopithecus* and *Homo erectus* before *Homo sapiens* appeared.

Speciation: the formation of a new species, when groups of a species are separated and change over time (geographical isolation).

Blood groups and heredity: the ABO blood group is controlled by three alleles (Iᴬ, Iᴮ and i). Iᴬ and Iᴮ are dominant over i, and Iᴬ and Iᴮ together show co-dominance (group AB). Group O has the genotype ii. A child's blood group can therefore help to show who cannot be the parent, but it cannot prove who is the parent.

Why variation helps: when the environment changes, some individuals with useful variations survive and breed. Asexual reproduction gives little variation, so a species of clones can be wiped out by a single new disease.

Teaching angle: use beads or coloured paper pieces for Mendel's crosses and a Punnett square, so that students see the ratios. Use the chart of homologous forelimbs and a family pedigree chart for discussion.

Pedigree and probability

A pedigree is a family chart that follows a trait through generations. Squares stand for males and circles for females; shaded symbols show the person with the trait. Probability helps to predict outcomes: for each birth the chance of a boy is 1/2. For two independent events the chances are multiplied, so the chance of two boys in a row is 1/2 × 1/2 = 1/4. In a Tt × Tt cross the chance of a dwarf (tt) is 1/4. Remember that these are chances for each child and not a guarantee for a family.

Exam traps

• Genotype (TT, Tt) is not phenotype (tall).
• Monohybrid phenotypic 3:1, genotypic 1:2:1; dihybrid phenotypic 9:3:3:1.
• Homologous organs have a common origin; analogous organs have the same function only.
• Sex is decided by the sperm, not the egg; the egg always carries X.
• Lamarck (use and disuse) is not Darwin (natural selection).
• De Vries proposed mutation; he also helped rediscover Mendel's work.
• Down syndrome is trisomy 21 (47); Turner is XO (45); Klinefelter is XXY (47).
• Colour blindness and haemophilia are X-linked recessive traits.

One-liners

• 1. Mendel is the Father of Genetics.
• 2. Mendel used the garden pea, *Pisum sativum*.
• 3. A test cross uses a homozygous recessive parent.
• 4. Humans have 23 pairs of chromosomes.
• 5. Females are XX and males are XY.
• 6. The Y chromosome comes from the father.
• 7. Haemophilia is a disorder of blood clotting.
• 8. *The Origin of Species* was written by Darwin.
• 9. Forelimbs of a human and a bat are homologous.
• 10. The appendix is a vestigial organ in humans.
• 11. *Archaeopteryx* is a link between reptiles and birds.
• 12. Miller and Urey made amino acids in a laboratory apparatus.

Practice questions

1. Who is known as the Father of Genetics?

   1. Gregor Mendel
   2. Hugo de Vries
   3. Lamarck
   4. Charles Darwin
   Answer

   A. Gregor Mendel

   Mendel formulated the basic laws of inheritance using pea plants.

2. Mendel conducted his experiments on

   1. maize
   2. fruit fly
   3. mango
   4. garden pea
   Answer

   D. garden pea

   The garden pea, Pisum sativum, has clear contrasting traits.

3. The phenotypic ratio of the F2 generation in a monohybrid cross is

   1. 3:1
   2. 1:1
   3. 1:2:1
   4. 9:3:3:1
   Answer

   A. 3:1

   Three dominant to one recessive; 1:2:1 is the genotypic ratio.

4. The phenotypic ratio of the F2 generation in a dihybrid cross is

   1. 9:3:3:1
   2. 3:1
   3. 1:2:1
   4. 1:1:1:1
   Answer

   A. 9:3:3:1

   Four phenotypes appear in this ratio.

5. How many pairs of chromosomes are present in a human body cell?

   1. 22
   2. 24
   3. 46
   4. 23
   Answer

   D. 23

   There are 46 chromosomes, which means 23 pairs.

6. The sex chromosomes of a human male are

   1. XO
   2. XX
   3. YY
   4. XY
   Answer

   D. XY

   Males have one X and one Y.

7. The sex of a human baby is decided by

   1. the age of the mother
   2. the egg of the mother
   3. the sperm of the father
   4. the diet of the mother
   Answer

   C. the sperm of the father

   The egg always carries X; the sperm carries X or Y.

8. A cross of an organism of unknown genotype with a homozygous recessive parent is called

   1. test cross
   2. back mutation
   3. monohybrid cross
   4. reciprocal cross
   Answer

   A. test cross

   It reveals whether the dominant individual is TT or Tt.

9. Colour blindness is a

   1. autosomal dominant trait
   2. trait caused by a virus
   3. sex-linked recessive trait
   4. Y-linked dominant trait
   Answer

   C. sex-linked recessive trait

   The gene is on the X chromosome and is recessive.

10. Down syndrome is caused by an extra copy of chromosome

    1. X
    2. 13
    3. 18
    4. 21
    Answer

    D. 21

    It is called trisomy 21.

11. Who proposed the theory of natural selection?

    1. Charles Darwin
    2. Mendel
    3. Lamarck
    4. Weismann
    Answer

    A. Charles Darwin

    Darwin explained evolution by natural selection.

12. The theory of use and disuse was proposed by

    1. Lamarck
    2. Oparin
    3. Darwin
    4. Wallace
    Answer

    A. Lamarck

    Lamarck believed that acquired characters are inherited.

13. Which pair of organs is homologous?

    1. Eye of an octopus and eye of a cat
    2. Leg of a frog and tail of a fish
    3. Wing of a bird and wing of a butterfly
    4. Forelimb of a human and flipper of a whale
    Answer

    D. Forelimb of a human and flipper of a whale

    They have the same basic bone plan but different functions.

14. Which of the following is a vestigial organ in humans?

    1. Kidney
    2. Heart
    3. Appendix
    4. Liver
    Answer

    C. Appendix

    The appendix has lost its original function.

15. The fossil Archaeopteryx is a connecting link between

    1. fish and amphibians
    2. reptiles and birds
    3. amphibians and reptiles
    4. birds and mammals
    Answer

    B. reptiles and birds

    It had feathers like birds and teeth and a tail like reptiles.

16. The Miller-Urey experiment produced

    1. DNA
    2. chlorophyll
    3. amino acids
    4. fossils
    Answer

    C. amino acids

    Sparks passed through simple gases gave amino acids.

17. A tall pea plant (Tt) is crossed with a dwarf plant (tt). The ratio of tall to dwarf plants in the offspring is

    1. all dwarf
    2. all tall
    3. 3:1
    4. 1:1
    Answer

    D. 1:1

    Tt × tt gives 1 Tt : 1 tt.

18. When two heterozygous tall plants (Tt × Tt) are crossed, the percentage of dwarf plants is

    1. 25%
    2. 50%
    3. 0%
    4. 75%
    Answer

    A. 25%

    The genotypes are 1 TT : 2 Tt : 1 tt, so one in four is tt.

19. The chance that a couple's third child will be a girl, if the first two were girls, is

    1. 100%
    2. 50%
    3. 25%
    4. 75%
    Answer

    B. 50%

    Each fertilisation is independent, so it is 50% every time.

20. A man with colour blindness marries a normal woman who is not a carrier. Their sons will be

    1. normal
    2. half carriers
    3. all colour blind
    4. half colour blind
    Answer

    A. normal

    Sons get the Y from the father and a normal X from the mother.

21. Wings of a bat and wings of an insect are examples of

    1. vestigial organs
    2. rudimentary genes
    3. analogous organs
    4. homologous organs
    Answer

    C. analogous organs

    They have the same function but a different structure and origin.

22. In peppered moths, dark moths became common near factories because

    1. dark moths were produced by industrial smoke
    2. light moths migrated to cities
    3. the moths used their wings more
    4. birds could see the light moths more easily on dark trees
    Answer

    D. birds could see the light moths more easily on dark trees

    This is natural selection, not inheritance of acquired traits.

23. Weismann's experiment of cutting the tails of mice for many generations showed that

    1. tails grow back in mice
    2. tail length is a dominant trait
    3. acquired characters are not inherited
    4. mice evolve long tails
    Answer

    C. acquired characters are not inherited

    The young mice were still born with tails.

24. An individual with the genotype Tt is

    1. heterozygous
    2. homozygous recessive
    3. a hybrid of two species
    4. homozygous dominant
    Answer

    A. heterozygous

    The two alleles are different.

25. In which situation does the child have 47 chromosomes?

    1. Turner syndrome
    2. a normal female
    3. a normal male
    4. Klinefelter syndrome
    Answer

    D. Klinefelter syndrome

    XXY male has 47; Turner XO has 45.

26. Which of the following correctly shows the sex chromosomes in birds?

    1. Male XY, female XX
    2. Male ZZ, female ZW
    3. Male XX, female XY
    4. Male ZW, female ZZ
    Answer

    B. Male ZZ, female ZW

    In birds the female is the heterogametic sex.

27. In some turtles and crocodiles, the sex of the young is decided by

    1. the sperm type
    2. the age of the mother
    3. the number of eggs
    4. incubation temperature of the eggs
    Answer

    D. incubation temperature of the eggs

    There are no sex chromosomes of the human type in these reptiles.

28. Which scientist, along with Correns and Tschermak, rediscovered Mendel's work?

    1. Hugo de Vries
    2. Haldane
    3. Miller
    4. Lamarck
    Answer

    A. Hugo de Vries

    They independently rediscovered it in 1900.

29. A plant with genotype RrYy forms how many types of gametes?

    1. 2
    2. 6
    3. 4
    4. 16
    Answer

    C. 4

    RY, Ry, rY and ry by independent assortment.

30. The base that pairs with adenine in DNA is

    1. uracil
    2. thymine
    3. cytosine
    4. guanine
    Answer

    B. thymine

    A pairs with T, G pairs with C.

31. Which of the following traits was NOT studied by Mendel?

    1. Seed shape
    2. Body colour of fruit flies
    3. Pod colour
    4. Flower position
    Answer

    B. Body colour of fruit flies

    Fruit flies were used much later by Morgan.

32. Statements: 1. Sperm decide the sex of the child. 2. Eggs may carry X or Y.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Eggs always carry X.

33. Statements: 1. Phenotype is the visible feature. 2. Genotype of a tall plant is always TT.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    A tall plant may be TT or Tt.

34. Statements: 1. Haemophilia is an X-linked recessive disorder. 2. Turner syndrome is XXY.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Turner syndrome is XO; XXY is Klinefelter.

35. Statements: 1. Fossils help in studying evolution. 2. Homologous organs show common ancestry.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements are correct.

36. Statements: 1. Lamarck's theory is based on natural selection. 2. Acquired characters are inherited, according to Darwin.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    Lamarck proposed use and disuse; Darwin did not accept inheritance of acquired characters.

37. Statements: 1. Mendel's law of segregation says each gamete carries one allele of a gene. 2. A recessive trait appears in the F1 of a pure cross.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    The recessive trait is hidden in F1 and appears in F2.

38. Statements: 1. Variation arises from sexual reproduction. 2. Mutations can create new variations.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are sources of variation.

39. Statements: 1. Homozygous individuals have two identical alleles. 2. A test cross uses a heterozygous recessive parent.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    The test parent is homozygous recessive.

40. Statements: 1. A pure tall plant crossed with a pure dwarf gives only tall plants in F1. 2. This is explained by the law of dominance.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

41. Statements: 1. Fossils in deeper layers are generally older. 2. Carbon dating is used to estimate the age of fossils.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements are correct.

42. Statements: 1. Mutation theory was given by Hugo de Vries. 2. Wallace had no similar idea to Darwin.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Wallace independently reached ideas close to natural selection.

43. Statements: 1. Mothers decide the sex of their child. 2. Drone honey bees form from unfertilised eggs.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    The father's sperm decides sex in humans; drones develop from unfertilised eggs.

44. Statements: 1. Analogous organs have a common origin. 2. Wings of an insect and a bird are analogous.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Analogous organs differ in origin and are only similar in function.

45. Which of the following is NOT a part of Darwin's ideas?

    1. Variation
    2. Struggle for existence
    3. Survival of the fittest
    4. Inheritance of acquired characters
    Answer

    D. Inheritance of acquired characters

    That idea belongs to Lamarck.