Fetal Development

• Create a timeline outlining the stages of human development

• Label a diagram of the stages of human development

KEY WORDSImplantation Embryo Trimester

Fetus

Haploid gametes contain 23 total chromosomes – 22 regular chromosomes and 1 sex chromosome.

Two types of sex chromosomes- called X and Y

• Sperm can have either one X OR one Y.

• Eggs always have one X chromosome.

• Every male body cell has one X and one Y.

• Every female body cell has two X.

MEIOSIS– MALES

46 ChromosomesDIPLOID 2n

HAPLOID 1n

x y

xx yyxx yy

Female sperm Male sperm

MEIOSIS– FEMALES

46 ChromosomesDIPLOID 2n

HAPLOID 1n

x x

xx xxxx xx

Female egg

x xx

y

Female zygote Male zygote

x xx

y

Egg and sperm meet in the oviduct - fertilization.

Sex is determined by the male at fertilization.

Nuclei are joined to produce a single diploid cell called a zygote.

2 stages of development - fertilization to birth:1. Embryo stage: first eight weeks.2. Fetus stage: from eight weeks to birth.

1. Embryo StageZygote implants in endometrial lining and begins togrow (divide) by mitosis for first eight weeks.

Once implanted – called an embryo.

Cleavage: mitotic divisions begin, changing zygote to a multicellular embryo.

2. Fetus StageDeveloping embryo forms bone cells around the ninth week of pregnancy.

Once bone forms - called a fetus until born.

Placenta - exchanges waste and nutrients through umbilical cord.

Amnion – protective sac filled with fluid.

Pregnancy can also be divided in 3 trimesters:

1st trimester – conception to 3rd month

• development of all body systems and organs.• sex can be determined• embryo movements not perceptible to the

woman. she may not know that she is pregnant

• At end - fetus weighs 28 grams, 7.5 mm long.• fetus is particularly sensitive to alcohol,

smoking and other drugs.

1st trimester

2nd trimester – 4th to 6th months

• period of rapid and continuous growth.• Only brain and lungs still developing.• embryo movements can be felt.• At end - fetus weighs 650 grams, 30 cm long.

• If born prematurely:Might survive with great medical assistance. Most do not survive.

2nd trimester

3rd trimester – 7th to 9th (birth) month

• fetus kicks, stretches and moves freely.• Eyes sense light, fingerprints have formed.• At end – fetus weighs 3300 grams, 35 cm long. • Dangers include complications during delivery

and lifestyle choices.

3rd trimester

And then, before you know it...

• Zygote at fertilization.• Embryo at implantation to endometrium.• Fetus after bone development.• Baby at birth.

• Fertilization to birth can be separated into 2 stages – embryo and fetus.

• Pregnancy can be separated into 3 trimester.

Homologous chromosomes are similar but NOT identical.

Homologous means that the chromosomes contain the same types of genes.

Gene - piece of DNA with instructions for a specific trait – hair colour, height, eye colour…

A diploid cell will contain two variations of each gene.

1 – mom1 – dad