chapter 19: heredity

Question 1

what are chromosomes?

Answer 1

• a rod-like structure visible in the nucleus during cell division
  > it is made up of the molecule deoxyribonucleic acid
• DNA in chromosomes carries the heredity information for making new organisms
  > each chromosome may carry many genes along its length

Question 2

what are genes?

Answer 2

• a gene is a unit of inheritance, born on a particular locus (locus) of a chromosome
  > it’s small segment of DNA in a chromosome that controls a particular characteristic or protein in an organism
• each gene has a specific function
  > there is a gene that determines height, colour and shape of plants
• the place on the chromosome where the gene is located is called the gene locus

Question 3

what are alleles?

Answer 3

• different forms of the same gene
  > occupy the same relative positions on a pair of homologous chromosomes

eg: height of pea plant has two alleles: short and tall

dominant allele: T
recessive allele: t

Question 4

what are homologous chromosomes?

Answer 4

• exist in pairs
  > one chromosome comes from the male parent and the other from the female parent
  > similar in shape and size
  > have exactly the same order or sequence of gene loci, alleles in those gene loci may not be the same

Question 5

what is phenotype?

Answer 5

• refers to the expressed trait in an organism
  > eg, the outward appearance of an organism
• the phenotype of an organism is the result of its genes and the effects of its environment

> eg. tallness in pea plants is a phenotype

Question 6

what is genotype?

Answer 6

a genotype is a genetic make-up (pairs of alleles) of an organism
> combination of genes in an organism

• an organism is HOMOZYGOUS for a trait if two alleles controlling the trait are identical
  > tt or TT
• an organism is heterozygous for a trait if the alleles controlling the trait are different
  > Tt

Question 7

what is a dominant allele?

Answer 7

• it expresses itself and gives the same phenotype in both homozygous and heterozygous conditions
• eg. tall plants have TT or Tt genotype

Question 8

what is a recessive allele?

Answer 8

• it does not express itself in the heterozygous condition
  > expresses itself only in the homozygous condition
  > eg. pea plants will only be dwarf if they have the tt genotype

Question 9

what is co-dominance?

Answer 9

co- dominance results when the two alleles controlling a trait both express themselves in the organism

Question 10

what chromosomes do males and females have?

Answer 10

• females have one pair of rod-like sex chromosomes
  > usually referred to as the X chromosomes
• each body cell in the male only has one X chromosome
  > the other is a hooked shaped chromosome; Y chromosome
• males have XX genotype and the male XY
• the other three pairs of homologous chromosomes are called autosomes
  > autosomes are chromosomes in a cell other than the sex chromosomes

Question 11

how many pairs of chromosomes do we have and what chromosomes do gametes contain?

Answer 11

• we have 22 pair of autosomal chromosomes
  > and a pair of sex chromosomes
• female eggs contain an X chromosome each
• males produce two types of sperms
  > one containing X chromosome and another Y chromosome

Question 12

what is continuous variation?

Answer 12

continuous variation is brought about by the additive effect of many genes
- contains a range of phenotypes
- controlled by many genes
- affected by the environment
egs. skin colour, height and intelligence

Question 13

what is discontinuous variation?

Answer 13

• contains a few clear cut phenotypes
• controlled by one or few genes
• easily distinguishable and not affected by the environment
  egs. roll tongue, wing length and blood types

Question 14

what is mutation?

Answer 14

• it is a sudden random change in the structure of a gene or in the chromosome number

Question 15

what is gene mutation?

Answer 15

• gene mutation produces variation between individuals as it results in new alleles of genes
• if gene mutation occurs during gamete production, the resulting genetic change can be inherited by the offspring
  > dominant mutations are easily detected whereas recessive ones may not be detectable for generations
• mutations that take place in the body cells other than gametes are called SOMATIC MUTATIONS
• somatic mutations are responsible for some kinds of cancer
  > somatic mutations cannot be passed on from parents to their offspring

Question 16

what is sickle-cell anaemia?

Answer 16

• it is an example of gene mutation
  > the mutation changes the structure of the gene
  > which is a change in the sequence of bases or nucleotides in the DNA
• the gene controlling haemoglobin production is involved in sickle-cell anaemia
• the mutated gene produces haemoglobin S (HbS), which is almost the same as normal haemoglobin A (HbA) except in one amino acid
  > this causes a change in the three-dimensional shape of haemoglobin
  > HbS molecules clump together, making the cell sickle-shaped
• mutated gene is recessive
  > only expressed in the homozygous recessive condition
  >people who are homozygous recessive have abnormal haemoglobin in their red blood cells
• when oxygen concentration in their blood decreases, the red blood cells become sickle-shaped
  > interferes with the oxygen carrying property of the red blood cells and also makes them fragile
  > fatal

Question 17

why are sickle-cell anaemia common in areas affected by malaria?

Answer 17

• individuals who are heterozygous for sickle-cell allele suffer less from the attack of malaria
  > because a small percentage of their red blood cells are sickle-shaped
• they are more resistant to malaria than individuals who have two copies of normal haemoglobin allele
• individuals who are homozygous for the sickle-cell allele have shorter life spans
  > heterozygous individuals have a better chance of surviving and reproducing in regions where malaria is prevalent
  > do not full contract either disease
  > this ensures that the recessive allele persists in the population

Question 18

what causes down’s syndrome?

Answer 18

chromosome mutation
- humans normally have 46 chromosomes
> some people have an extra chromosome; 47 chromosomes in their body cells
- normally, zygotes with extra chromosomes would fail to develop
>one of the few exceptions is a zygote with an extra copy of chromosome 21

Question 19

what are mutagenic radiations?

Answer 19

• the rate of spontaneous mutation is usually very low
• the rate of mutation is greatly increased with the presence of certain agents in the environment
  > mutagens
• ultraviolet light, and alpha, beta and gamma radiations are mutagenic
• some chemicals, if present in certain concentrations, are mutagenic

Question 20

what is natural selection?

Answer 20

• variations in organisms may arise due to mutation
• competition for food and space occurs among these different varieties of organisms
  > nature selects those varieties that are more competitive, more resistant to diseases and better adapted to the changes in the environment
  > so survive and reproduce their kind
• the other varieties that are suceptible to diseases or environmental changes may die
  > natural selection
• mutation provides new genes or alleles for natural selection to operate on
• after thousands or millions of years, more beneficial qualities (genes or alleles) may accumulate in a species
  > the new breed of organisms may be better adapted to their new environment
  > may even change so much that they become a new species which differs from the original stock which they have evolved
  >