Chapter Three:

1
Chapter Three

• Heredity and Environment

PowerPoints prepared by Cathie Robertson,
Grossmont College
2
The Beginnings of Human Life

• Development starts at the time of conception.
• During conception, the reproductive cells are
  fused together.
• Gametes Sperm and ovum
• The fused cell is called the zygote.

3
Zygote and Genotype

• Gametes are haploid structures which combine to
  form a diploid zygote
• Each cell (except gamates) has 23 pairs of
  chromosomes.
• Chromosomes are double-helical or spiral
  structures with two strands.
• Each strand has genes from one parent that pairs
  up with same genes on the second strand from
  another parent.

4
(No Transcript)
5
(No Transcript)
6

• Genes carry instructions to make all the proteins
  that guide development.
• Genes are the basic unit for the transmission of
  heredity instructions.
• Every gene is a separate section on the
  chromosome and each contains instructions for a
  specific protein.
• Genes are made up of DNAs (deoxyribonucleic
  acid).

7

• DNAs consist of the following four chemicals
  adenine (A), thiamin (T), guanine (G) cytosine
  (C)
• DNA contains the instructions for building
  proteins that control the structure and function
  of all the cells that make up the body.
• Four types of pairings are possible A-T, T-A,
  C-G, and G-C.
• A sequence of three pairings constitutes a gene.

8
(No Transcript)
9
Sex Determination Sex Ratio

• In 22 of 23 pairs, the two chromosomes are very
  closely matched but not identical
• some genes come in slight, normal variations
  called alleles
• E.g. hair color black and brown eye-color blue
  and brown, skin color black, brown and white.
• The 23rd pair is different
• in females, it is designated XX
• in males, it is designated XY

10
Sex Determination and Sex Ratio, cont.

• Females always contribute one X
• Males will have 1/2 of the sperm contributing an
  X and the other half contributing a Y
• The chances of having a male or a female are
  5050.

11
Gene Expressions Interactions

• Most genetic traits are multifactorial
• Environment and other genes (polygenic)
• Some genes control and regulate the functioning
  of other genes
• Additive genes every gene contributes equally to
  the genetic expression (like height).
• Dominant recessive genes