CHAPTER 2: PRENATAL DEVELOPMENT & THE BIRTH PROCESS:
THE PRENATAL PERIOD:
DOMINANCE & RECESSIVITY:
2 Genes on each pair of chromosomes are jointly responsible for the determination of
hereditary characteristics. This gene pair is known as an allele.
- When 2 alleles are homozygous (identical) in terms of the characteristics they control,
the receiving individual will actual show that particular characteristic.
- BUT when the 2 alleles contain different instructions (heterozygous) we look at the
principle of domination and recessivity.
A dominant gene determines the overt/observable characteristics (phenotype)
The recessive gene characteristics do not manifest but is still part of the underlying make-up
(genotype) and can still be passed down.
, Many diseases are the product of recessive genes.
Children who inherit the dominant allele will always develop the disorder.
They seldom live long enough to transfer it to the next generation ___ except for
Huntington’s Disease.
SEX DETERMINATION, SEX-LINKED CHARACTERISTCS & POLYGENIC INHERITENCE:
-Gender is determined by a sex chromosome called a gamete.
-The sex chromosomes = the 23rd pair of chromosomes
-Women have 2 similar X chromosomes and men have 1 X and 1 smaller Y chromosome.
THEREFORE, (Women = XX) (Men = XY)
-Gender of a baby is determined by the father’s sperm.
o If a sperm carrying the X chromosome penetrates and fertilizes the egg first – the
fertilized egg will have an XX chromosome and will therefore be a GIRL.
o If a sperm carrying the Y chromosome penetrates and fertilizes the egg first – the
fertilized egg will have an XY chromosome and will therefore be a BOY.
The chances of having a boy or a girl are theoretically equal, however there are slightly more
boys born than girls.
THE PRENATAL PERIOD:
DOMINANCE & RECESSIVITY:
2 Genes on each pair of chromosomes are jointly responsible for the determination of
hereditary characteristics. This gene pair is known as an allele.
- When 2 alleles are homozygous (identical) in terms of the characteristics they control,
the receiving individual will actual show that particular characteristic.
- BUT when the 2 alleles contain different instructions (heterozygous) we look at the
principle of domination and recessivity.
A dominant gene determines the overt/observable characteristics (phenotype)
The recessive gene characteristics do not manifest but is still part of the underlying make-up
(genotype) and can still be passed down.
, Many diseases are the product of recessive genes.
Children who inherit the dominant allele will always develop the disorder.
They seldom live long enough to transfer it to the next generation ___ except for
Huntington’s Disease.
SEX DETERMINATION, SEX-LINKED CHARACTERISTCS & POLYGENIC INHERITENCE:
-Gender is determined by a sex chromosome called a gamete.
-The sex chromosomes = the 23rd pair of chromosomes
-Women have 2 similar X chromosomes and men have 1 X and 1 smaller Y chromosome.
THEREFORE, (Women = XX) (Men = XY)
-Gender of a baby is determined by the father’s sperm.
o If a sperm carrying the X chromosome penetrates and fertilizes the egg first – the
fertilized egg will have an XX chromosome and will therefore be a GIRL.
o If a sperm carrying the Y chromosome penetrates and fertilizes the egg first – the
fertilized egg will have an XY chromosome and will therefore be a BOY.
The chances of having a boy or a girl are theoretically equal, however there are slightly more
boys born than girls.
